I am cleaning an servicing sn 3198. The case does not have white ring. I have seen that very early curtas came without this ring if I am right... Does someone know when they started to put it?
Thanks!

This is the Curta I put a ring but I would like to know if I need to remove it. It belonged to Peter Fraiser, applied mathematician and a very interesting guy.

The case was designed to use with an O-ring, I think. So it's likely that the oldest known Curtas simply lost the rings (with a higher possibility than younger ones...).

But we will never solve all the Curta secrets