The little FP-45 (Flare Pistol was the cover name) is interesting. The book is "The FP-45 Liberator Pistol" by R.W. Koch, but I think it is out of print.
The gun is a single shot, made from stampings, castings and welds. The barrel is rolled sheet metal. There is space in the butt for 10 rounds of .45 ACP; not a magazine, just some ammo.
I have fired mine and it is accurate enough to hit a can at 7 yards. Smooth bore, of course, but not considered a "smooth bore pistol" by BATF, so no extra legal complications.
There seem to be few records of disposition or any indication what happened to most of them. One veteran told me of kicking thousands of the boxes out of a C-47 over France, but other authorities say they never went to France and French ex-maquis say they never saw them. Not necessarily contradictory, but you would think someone would find one or two (or get hit on the head) if they were just dropped.
Other sources claim they went to the Philippines, but same story there - they never saw them.
So... Destroyed? Dumped in the ocean? Who really knows? But they are scarce in this country, especially if with the brown waxed box, the wooden stick (extractor) and the instruction sheet. All, unfortunately, have been repro'd.
Anyway, if they were used, neither our allies nor our former enemies seem to have known about it.
Yes, I think the pistol is worth $400, maybe $500 if the complete kit.
As to making one, it would not be easy to make it the way GM did, since heavy duty sheet metal rolling and stamping machines and spot welders are not lying about in the average gun shop. To make something like it would be easy.
Jim