Looking at the fastener on your 1941 data plate (I didn't see it at first), it looks original with the small combo clutch/hex head. It might be possible that the body has been switched out sometime in the past 80+ years to a different chassis. A lot can happen in that time period.
Likely more study would be necessary to nail it down. It used to be that GM would send you an information packet if you sent them your serial number, but I don't think that's available now. I do have a contact somewhere for a lady who works at the GM Heritage Center. If you'd like I could track that down for you.
