It has the internal extractor, disassembly cut-out for the finger, aluminum magazine follower, and SN in the 777xx range. That would seem to indicate it was made between 1954 and 1957. I believe the internal extractor was dropped in 1962 and the finger cut-out in 1959. Can anyone tell me when they quit using the aluminum magazine follower?
On the frame and slide (left side) it has a rampant lion on its back, then a vertical line and "P.V". On the barrel, it has these same markings, plus a "cartouche" shaped somewhat like a pineapple, with E L G inside of the outline. The top of the "pineapple" could be a crown. What do these indicate?
I would guess economics lead to the follower change, either that, or the quality of plastics improved to the point where FN trusted it. It could be either one, or both, reasons, but it's likely one or the other. Money makes the world go round.
As far as the markings go, they are just proofmarks from the maker that satisfy European standards for quality and workmanship.
You can roll a turd in peanuts, dip it in chocolate, and it still ain't no damn Baby Ruth.
What you're describing is a standard Hi Power from the 1950's with proper Belgian proof marks. Sorry, I don't recall when they switched to the plastic follower...I wanna say late '60's perhaps.