MMM, Manufacturas Metálicas Madrileñas (Madrid metallic manufactures) was a private firm who made 12,7 x 99 ammunition for the Spanish Air Force in the '60s. The ammo was used in US-origin airplanes. Functional types, as far as I know, were only API M8 and API-T M20, with US color codes.
In 1968 the company went bankrupt (not without controversy) and Santa Bárbara took over the ammunition manufacturing line. SB inherited also the remaining stock of MMM brass.
But mixed with MMM rounds sometimes these MFS cartridges were found: lacquered steel cases in brownish-green colour, brass primer, 3 stakes, black primer seal. The bullets were similar to the MMM ones (M8 and M20).
There is evidence that the Spanish Air Force fired these MFS rounds too, so it’s not that MMM and MFS became mixed at the SB warehouses. What is not clear is what hungarian eastern block ammunition had to do with spanish production by a private firm.
I have seen on the internet that MFS still exists and makes ammo, but only in commercial rifle and pistol calibers.
Any more information on the 12,7 mm MFS subject would be very welcome.