The Argentine factory FMAP-DGFM was first set up to produce a copy of the Colt 1911, and in 1954 they began production of the PAM-1 (after demonstration of the first prototype in 1950). The PAM-1 was a copy of the American M3A1 "Grease Gun" chambered for 9x19mm Parabellum. Production ran until 1961, at which point the factory switched to production of the new FAL rifle for Argentine forces. By that time, about 33,000 had been produced.

Almost as soon as production ceased a process of conversion and upgrade began, creating the PAM-2. This involved adding a grip safety lever to the back of the magazine well. This prevented the gun from being cocked or fired, and it was most likely done to prevent cases of the guns firing when dropped. About half of the total production was converted to the new configuration, and a second small batch of about 1,100 PAM-2s were manufactured to this pattern in 1969. Most of the examples outside Argentina are PAM-2 conversions, as they were captured by British forces in the Falklands.

By the 1970s, the PM-3 had been officially replaced with the PAM-3, better known as the FMK submachine gun. The Grease Guns remained in service alongside the FMKs until the 1990s, however.

Many thanks to the Royal Armouries for allowing me to film and disassemble this rare submachine gun! The NFC collection there - perhaps the best military small arms collection in Western Europe - is available by appointment to researchers:
https://royalarmouries.org/research/national-firearms-centre/

You can browse the various Armouries collections online here:
https://royalarmouries.org/collection/