1951-1952 date for the movement, cased in the watch lightly later.

Possibly original I think!

For 1949-50 it is possible, the original buckle was not signed as the crowns were not signed. If the buckle was signed, it would have had a small triangular JL logo in the middle of the buckle but that probably only started in the mid 1950s.

Looks good!

What is stamped on the inside and outside of the back?

The reference would be E21004. I'd need the movement serial number to date it.

It would date to approx. 1966.

Hard to say! It could be a french market watch. A lot of weird stuff was done in the 1970s using movements made by other manufacturers.
Either way it's not super collectible even if real. Any indication it's made in France?

They are counterfeited, but yours to my eyes, looks right. The serial # should be accurate.

Mid 1950s. You will need shot of the movement with serial number and inside the case back for further information.