The one missing part was the flange of the winding opening in the front of the case. This flange or escutcheon not only protects the hole but also lends support to the handle during winding. The original part perhaps buckled and broke from the repeated circling load stress. Probably was made of stamped thin sheet metal.
The outline of the original part could still be seen on the outer case, a slight imprint and a different color of the leathercloth where it was protected by the flange (at least for the first part of its life). Using this outline and taking some guesses on likely dimensions, a new part was modeled in 3D that would be suitable for printing. For this I used the free to use Sketchup Maker version. It takes some getting used to, as far as I can tell it is a quite powerful 3D engine with relatively limited and simple editing tools on top.
The maker version of Sketchup has only a few file format options, but the model can be exported as a Collada 3D file (.dae file). As advised by Shapeways, this Collada file can then be loaded in the (free) Meshlab tool. This tool is all about the mesh and it offers many functions for inspecting or fixing the mesh. Having taken some care to have a closed model in Sketchup, steered well clear of all that and just exported the part as an STL file suitable for the 3D print shop.
Not being all that sure of the dimensions and proper fit, ordering it right away in metal seemed a step too far. Settled for the regular black plastic version, much cheaper and easier to drill and file to size. Arrived in due time and looked very clean and of course 'sandy'. As they explain very clearly, the Nylon part is made on an SLS machine so consists of lots of small plastic spheres that are laser-fuzed together. This also means that sanding a surface will not yield a smooth finish - just more 'sand'.
When painting the part, the first layer (or two) get sucked into the part. This probably has as side-effect that the part gets quite a bit stronger, as the fuzed spheres are now held more rigidly under load. It actually took 3 layers of paint (and some sanding in between) to get a decent looking part.
Unfortunately I got the pitch diameter of the screws slightly too small, so carefully drilling the holes larger was needed to get a good fit. During fitting to the case, one of the holes in the case needed to be re-drilled. Otherwise it fits very snuggly in the slanted winding hole.
The central winding hole diameter could have been a bit smaller, to be a closer fit with the winding handle. Then it would flex a bit less when winding the spring, but it gives decent support as it is now.
All in all a reasonable replacement part. Now that I've made and fitted this one, I know how to modify the holes (make them slots) for a better fit. Also the flange could be designed a bit different to match the clasp a bit better. But for now I'll stick with this one. (Until it breaks, of course. Then we just print and paint another one... ;)
We'll see how it holds up, but 3D printing seems a viable route for replacement parts :-)