In both types of calculator there are two different types of pins used for carry operations: the "full carry" pin (part #10019) which is a small metal cylinder, and the "half carry pin" (part #10021), which is tapered on the end into a half cylinder. Both types of pins stick out of the counter and accumulator dials (parts #10043 and #10045), with the end result that there are four different types of numeral dials: split-gear wih half-pin (assembly #10203), nonsplit gear with full pin (#10204), nonsplit with half pin (#10205), and nonsplit with no pin (#10045).

Most of this design I understand. The split-gear versus nonsplit-gear distinction is to desynchronize the zeroing process for smoother operation, and the two dials without carry pins are the highest accumulator and counter digits respectively (and thus have no need for carry). But I'm flummoxed by the function of half pins versus full pins: their job is to actuate the carry levers (parts #10005 and #10006), which AFAICT don't much care about the shape of the object striking them. From engineering diagrams of the Type I assembly it looks like the half pins are used on the 4 lowest digits of both the accumulator and the counter, no pin on the highest (unsurprisingly), and full pins on the rest. What's the rationale for this design distinction?

Here is a picture to this interesting question... (from the Service Manual)



... but really no idea - can't see the functional difference at position 4 and 5:

15 15 16 16 17 18
15 15 16 16 17 17 17 17 17 17 18

Yup, that corresponds closely to both the picture on Rick Furr's Curta poster (which appears to be lifted directly from the service manual but with the numbers changed), and to the arrangement in the engineering diagram for the carriage subassembly first step (assembly #10243).

Interestingly, the Type II, according to its service manual, only uses the half carry pins, which argues in favor of the notion that there was no particular mechanical distinction between the two components, but I'm still curious about the design rationale for using a mixture of the two pin types.