I must say .010 sounds like the right number for the end float of starter and the one that is in there now makes a disengaging sound and the ring gear is new, and I do not want to damage it. BUT the end float on the one that came with the parts car is more like 3/8 of an inch, and it looks new inside; it was rebuild but by who? The washer on the end of the armature “I guess for end float” is steel; the steel shims sits below the copper armature, so I guess it should not matter and I have some washers just like it in different thicknesses and perhaps with one them that is about.020 that will pull things tighter and the 3/8s is kinda of the whole deal moving because the starter gear slides out so much on the end with that circlip and retainer set up.
In the past, I would make things like that out of leather because I felt they would last and help with the impact, but I am not sure what to do with this rebuild.
I guess put a shim on the end shaft and see how much the square drive sticks out the end and adjust accordingly.