My 1971 4.2L doesn't have a stamp on the right side of the picture frame. So either it was replaced or it was never there. I've read that in 71 the number was moved to behind the heater, but that may only apply to V-12 cars. No number there that I can see.

I'm assuming the picture frame is the correct location and was, oddly, replaced very early in the car's life. The last owner had the car for 42 years and didn't do it.

If the right side of the picture frame is the correct location, as I have seen in many photos, my question is, "Was it stamped before paint or after?" I'm curious if original stamps on cars not repainted would show paint down inside of the debossed numbers/letters.

 

MrVette,
I believe the chassis number was stamped before paint. If it was stamped after paint, the stamping process would break the paint surface and leave the metal picture frame exposed to the elements and it would rust in no time at all.
I have seen cars that have been repainted and the numbers almost completely covered. So, try to shine a light on the area from different angles and see if you can find evidence of the stamping being covered by paint.
If the previous owner had the car for 42 years, that leaves the first 9 years unaccounted for. That is plenty of time for something to happen and cause the picture frame replacement.
Good luck.