The most obvious solution is 99% probable and that would be your theory of stamps being applied at BSA prior to any work beginning on them. I stress that I am saying this not from a position of knowledge but a position of looking at probability given a) the fake market is recent and b) I have never seen this mentioned before even on that website, and if it was a possible to sell fakes with this it would have been done and seen. I guess it's only a matter of time😀