I believe Smith and Smith SAOTW had these in them by the late 1960s or early 1970s.
The linked article seemed to imply they were for issuing South Vietnamese but this was never the case.
Some folks had spent too many Sunday Mornings on the living room floor with the Sunday Funnies reading Terry and The Pirates and thought everyone in Laos was slaves of some warlord or the Dragon Lady Herself and that everyone in North Vietnam was an oppressed slave of communist leaders worse than Albania or North Korea ever had and their mindless minion Armies.
These Deer Guns were to Opresso Libre the Laotians and North Vietnamese folks by giving them a simple gun they could use with only comic book training. Now folks might balk at a smooth bore pistol with crappy sights, but if your best shooter was a Monkey or Bird crossbow the Deer Gun was a huge step up.
I suspect that one reason they were not deployed was because we had armed the Mountain Peoples of South Vietnam with Carbines grease guns and even M1 Thompsons.....and a large portion of them, about the time this Deer Gun manufacturing was going on Revolted and put on Red hankies. Suddenly the idea of purposefully arming folks we don't really know with ANYTHING that goes bang made a lot less sense.
BTW National Geographic's coverage of the Revolt was pretty much most American's first hint that their was a problem in Vietnam and that we were involved with troops on the ground (wearing funny Green French hats) that were getting shot at.
Meanwhile back to the original FP45. I have known two men that claim to have had one as teens in 1944-45, one Dutch and one Hungarian. The Dutch guy claimed his was used to shoot Germans on two occasions to obtain rifles the Hungarian claimed his gave his little gang of boys the courage to sneak onto a German airbase and damage Aircraft while the guard was off getting lunch. The Hungarian's may still be in the bottom of a stream where he threw it during their escape from the airbase after they were spotted and chased and he decided being caught with it might be a Bad Thing.
In 1981 just before Christmas I was at what might be described as a gun and knife show in Frankfurt BRD (FRG) at the Fifth Core HQ conference center. At a table was a guy selling FR45s in Sardine cans that were green in color and featured the old style key opener. The one can opened when I got their and the can he opened in front of a group of us contained a pistol with a grip full of ammo and a wooden dowel and a Comic strip. I have been told repeatedly by experts that the guns were in cardboard boxes and never packaged as these. There were at least twenty such cans on the table . Someone Somewhere Sometime canned Those Guns.
At the time I felt that buying one of those cans was a pretty good way to make my next assignment be to Ft. Levenworth, and not as an officer, so I passed. Chief WishIdas from that show were a couple of very nice original Luftwaffee Swords and a nice original Luger Snail Drum. I did buy an AK47 (not AKM) bayonet and sheath that, being as how no one had one, got a lot of OOOs and AHHHs at that time before the flood.
-kBob