Yes, 9B19 is a known and desirable date. Yes you can see the year digit was a 9 before the crude overstamp. Was it a 9 from the factory? Quite possibly. But equally, impossible to say for sure from those pictures. What it was once, is not what it is now, hence why in my opinion, I would pass - or at least not assign any more value than just another 37A alternator core.