During World War Two, the British government contracted for about 8,000 Ballester Molina pistols from HAFDASA in Argentina. They were produced between 1942 and 1944, and are easily identified by the application of a second serial number on the left side with a “B” prefix. The exact details of the contract are lost, but the British appear to have paid at least in part in steel (in very short supply in Argentina at the time) and also supplied the steel needed to make the pistols, probably via US Lend-Lease. Some of these Ballester Molina pistols were supplied to SOE agents or resistance organizations, while others remained unissued at the end of the war. Those unissued guns were purchased by InterArms in the 1950s, and exported to the US. The guns had not originally been given proof marks, but a British proof law introduced in 1955 required proofing when the surplus ones went to InterArms, and you can see those marks on this example.
Reference: The information for this video came primarily from Alex Gherovici’s monograph “Military Pistols of Argentina”.