After fiddling around with the part from the previous article, I think I might have a reverse engineered technical diagram. I still don't know enough about early 20th century mechanical design techniques to know if this is what they would have done, but it should be enough to at least remanufacture this part.
I also realized that I haven't actually described the part! There are two registers on the typical Monroe calculator, an upper register which indicates operation count (useful for multiplication and division) and a lower register which indicates total. There's a crank which, when turned one way, zeroes out the upper register, and when turned the other way, zeroes out the lower register. The part that I reverse engineered is shown in the original 1920 US patent 1,396,612 by Nelson White, "Zero setting mechanism" in Figure 5. In the patent, the part, 32, is described as follows:
So the next step might be to make an OpenSCAD file for the part, and put it on Thingiverse so that anyone can recreate the part. It probably can't be 3D-printed at this point, since it really needs to be a metal part. Even Shapeways, which can 3D print metal parts from stainless steel combined with bronze, can only achieve a 1mm detail, and this thing is much more detailed than that.
UPDATE: See the thing on Thingiverse.