.. then I am truly scared of the vintage market these days, especially if the rest of the watch passes your inspection.
My thoughts: My feeling is that this is ok. Wearing down of stamped casebacks happens all the time with vintage PP and this example does not strike me as terribly different from others that I have seen. Also, I don't believe the lack of a ref # is an issue, especially if your 1593 is from the early 1940s (which I think it is). If it is from the 1950s or beyond, then I would begin to worry, because these Hour Glasses do have stamped reference numbers. I would guess that the watch has been at least lightly polished at some point in the relative near future (last 15 years), which may explain the inconsistency in condition with that of the movement and caseback. If it comes on its original GF bracelet, this can be another reference point regarding condition (and frankly, a very strong indicator that this watch is all original).
My recommendations: 1. Feel comfortable with the vendor; 2. Order archive papers; 3. If need be, consult with John Naga from our forum (if you don't know him, message me and I'll send you his info) or other leading vintage PP expert.
Love this model by the way. It looks like you've decided to go with gold, but if you're also considering the platinum version, I know someone personally who has one for sale (diamond markers, GF bracelet, pm me if you'd like more info). Anyway, good luck with the hunt! Love this reference
Cheers,
Matt