I might be wrong about this, but one more point about the serial number bridge beeing altered:
Maybe it was ground down on a lathe and restamped, look at the two pictures and how low/shallow the screw sits in it's hole in comparison. First pic from OP, second provided by
@X vintage.
View attachment 643043
View attachment 643041
Now the easy explanation could be that the screw is just not properly seated, but that's also strange, given that compared to all the others screws they seem to be seated correctly.
Cheers,
Max
Click to expand...