In 1955-56 a new variant was added in the curta production: the silencing spring.
It is basically a spring which is wound around the groove in the 0 positioning disc with one end that is bent and passes under the non return pawl and enters its own specific hole (2mm) in the base (lower main casting).

The way it operates is simple: during the normal rotation of the main shaft, the spring keeps the non return pawl slightly lifted up, so that it loses its contact with the disc teeth and doesn’t produce its usual ticking. Yet, if we try the inverse rotation, the spring backs up and the pawl starts meshing again with the disc teeth, thus preventing, as it usually does, such inverse motion.
We do not exactly know how many silent curtas were prepared (no more than a few thousands, however), but there is a strange fact: some curtas were made with the hole in the base, but without the spring. Therefore, in operation they produce their normal ticking, they are not silent, but their base has the specific hole for the silencing spring.
According to my experience, I would say that, considering the curtas with this hole, only one out of eight or one out of ten also has the spring.
At a certain point, they might have stopped adding these springs. But why?
Were they taken away in a second moment? And who did that? Why?
Here is the data I have collected from my direct experience. The gaps are so large because I have only recently been attracted to this issue.

CI:
30029 without hole
-----------
32036 silent, now on eBay
33728 silent
37625 hole without spring
41681 hole without spring
-----------
46509 without hole

CII:
501528 without hole
-----------
504094 hole without spring (poster on eBay)
506547 silent
507157 silent (photo by Sebastian Dammers in this BB) https://www.flickr.com/photos/131105513@N02/16404073206
511272 hole without spring
-----------
514051 without hole

I would like to invite all curta owners to add further information after checking their curtas. Besides mentioning the serial numbers of the silent curtas they may have, they should also check if, in curtas whose serial number is close to the silent curtas range, there is a hole in the base. It’s easy: you only have to remove the base plate unscrewing the two counter sunk screws; it’s a 2mm hole and it’s under the non return pawl.
It can also be seen when looking laterally, but in order to see it well, you can lift the non return pawl with the tip of a screwdriver.
I am confident that all together we will be able to define stricter limits and understand more about this variant which is very interesting for collectors.

Perhaps this silent curta was not appreciated by everybody and at Contina’s they soon stopped putting the springs, even though they continued to mount the bases that had already been prepared with the hole?
Perhaps the springs were taken away by dealers upon requests of their customers who preferred to hear the typical ticking?
Perhaps the springs, whose correct operation depends on the right amount of friction between the spring and the disc, could no longer work correctly after some time or due to dust and so they were removed at the first problem or the first time the curtas were cleaned.

RoMan
http://www.curtaservice.it/

Interesting overview about the mysterious silent Curta!

To visualize the modified Lower main casting (the 2 mm hole) the following two pictures:


[ without the silent modification ]



[ with the silent modification ]


Is there a picture of the silent spring (not to confuse with loud summer )

I will check some relevant numbers...

For meticulous people (as I am) I want to add a further curiosity that I have noticed.
In CIs the spring is wound around the disc in a gripping way, that is the friction on the disc during normal motion tends to grip the spring around the disc.
In CIIs the spring is wound in the opposite way, so that the friction tends to loosen the spring.


[CI and CII silent springs]

Both ways of mounting the spring can cause problems.
In the gripping way, if the friction between the spring and the disc tends to increase in time, the spring might be drifted and finally come out of its hole in the base.
In the loosening way, the free end of the spring only needs to find the slightest unevenness in the disc groove to get stuck and come out of the groove (in order to avoid this, the free end of CII springs is bent outwards), but the same problem can arise as friction increases in time.
I prefer when springs are mounted in the gripping way, it is safer and its possible defects in my opinion are not so frequent and anyway can be avoided by slightly lubricating the disc groove.
They might have preferred it at Contina’s as well, but they were obliged to use the opposite mounting owing to the different zero positioning disc.
In the CII zero positioning disc (smaller) the roller indentation is deeper than the groove and, with a gripping spring, when this indentation passes under the fixed end of the spring, this slightly enters the indentation and, at the end of the indentation, the spring derails every time outside the groove.


RoMan
http://www.curtaservice.it/

I can confirm the range of serial numbers, both Curta Type I and II - no new or tighter borders found...

And all my Curtas without the spring.

Do you recreate these springs? I would be interested in some tests...

Sorry for being so late with my reply.
Yes Murff, springs are simple, they only need to be carefully calibrated with the right amount of friction on the disc, but I think they can surely be recreated.
And I also think that putting a spring in a curta that already has the hole shouldn't be considered a fake. What do you think?

... I think as long as a modification is declared (e.g. a silent spring) it's not a fake. For me a faked Curta is one with modifications that are not original (e.g. a new made demonstration model...) with the purpose to get more profit.

Over time there are some Curtas in circulation with some special modifications and only in rare cases you will be able to identify it for sure as a genuine Contina modification - or as a fake...

502852 > no hole origin: Frankfurt/Main, Germany
507157 > hole with spring origin: Barcelona, Spain
509135 > hole but no spring origin: Atlanta, USA

Many thanks to Murff and Sebastian for their cooperation.

Romano, thanks for that interesting info!

I can add one more:
506998 > Hole with spring. Origin: Zaragoza, Spain

I'll get my screwdriver out and have a look, interesting

I don't know whether this helps, but my Curta II, S/N 508965 has a hole but no spring.

Good luck with your research!

RoMan wrote:

In 1955-56 a new variant was added in the curta production: the silencing spring. It is basically a spring which is wound around the groove in the 0 positioning disc with one end that is bent and passes under the non return pawl and enters its own specific hole (2mm) in the base (lower main casting).

Something new about the mystery:



In an old general plan of the Curta Type 1 I discovered the mechanism (the spring) of the "silent Curta". The plan was last revised in
April 1953 - so the "silent Curta" option was realized very early. In the newer known plans this detail has disappeared again.

Thank you, Murff it's very interesting.
I wrote 1956-57 because the very few curtas that were found with the hole and the silencing spring date back to those years.
Yet, as a matter of fact, Contina must have decided about this modification at least back in 1953, since type II curtas with the
hole only, but without the spring, actually date back to 1953-54, so everything fits.
I'm also posting the updated list of silent curtas or curtas with the hole.
The reason why they didn't fit the springs in type II curtas with the hole right from the start still has to be explained.

CI
29840 no hole 1955
30029 no hole 1955
30840 hole and spring 1955
32036 hole and spring 1955
33458 hole and spring 1956
33728 hole and spring 1956
33890 hole and spring 1956
34168 hole and spring 1956
35628 hole 1956
36422 hole 1957
37625 hole 1957
41681 hole 1959
46509 no hole 1961
49039 no hole 1961

CII:
501528 no hole 1953/54
502852 no hole 1953/54
503624 hole 1953/54
503937 hole 1953/54
504094 hole 1953/54
504464 hole 1953/54
505540 hole 1955
505582 hole 1955
506547 hole and spring 1955
506998 hole and spring 1955
507157 hole and spring 1956
508965 hole 1956
509135 hole 1956
511272 hole 1958
511655 hole 1959
512346 no hole 1959
514051 no hole 1960

Here are two springs which were used for the silent mode:

I have the spring in my Curta ii.[/img]