I always thought , that I can only be sure to buy a 100% original piece, if I buy it from the first original owner, or his family.
Once I bought a 5513 from 1966 from the son of the original owner. He lived in the same small town like me. I knew him since decades.
So I was 10.000% sure, that this must be 1000000% original. And it was with box and papers.
A lot of times the watchmaker tried to make it waterproof , new crown, new tube, new crystal, new gaskets, but it was impossible.
The watchmaker told me, that he think the middle case was not original.
I talked to the original owner again , and he told me, that he almost forgot, that he have been in Hong Kong
some twenty years ago. In Hong Kong he had an accident , and the case was badly damaged. One lug was almost missing.
A watchmaker there repaired the watch in five days, with a replacement case. He even engraved the old serial numbers in the new case.
So even this 10000% Original first owner buy is not safe....