The 6536 you show is worn alot, I have always been under the impression that the back should be flat.
I was refering to the cheaper as less inaccurate while both actually are quite bad.
Assumed that the inserts in those watches (223 vs. 176) are of the same size, the difference in measured width looks to be in the bezels outer diameter.
While the bezel of 176 seems to be as wide as the actual case, the bezel of 223 looks to "hang over" the case, which leaves us with the question whether the case itself on 223 is 40 or perhaps smaller?
Just a thought worth checking out, awesome projekt btw & good luck!