A headstamp is the markings on the bottom of a cartridge case designed for a firearm. It usually tells who manufactured the case. If it is a civilian case it often also tells the caliber: if it is military, the year of manufacture is often added.

The headstamp is punched into the base of the cartridge during manufacture. A resource for identifying where the ammunition originated can be found at Cartridge Collectors.

The government-operated arsenal K.M. Polican marks number "3" at the 6 o'clock position, and the last two digits of the year at 12 o'clock.

Military cartridges have the manufacturer's head stamp at 12 o’clock, and can include a two or four digit year. The following list of manufacturer initials is taken verbatim from:

FMMAP S.F. Fabrica Militar de Cartuchos y Armas Portatiles de San Francisco ("Military Cartridge & Small Arms Factory - San Francisco") (1948-1954)

F.M.M.A.P. Fabrica Militar de Municiones y Armas Portatiles ("Military Munitions & Small Arms Factory") (1939–44).

I.M.P.A. Industria Metalurgica y Plastica Argentina ("Industrial Metallurgy & Plastics - Argentina") - Buenos Aires, Argentina.

There are companies that do not manufacture or assemble cartridges of their own but import or contract them from another manufacturer.
La Porteña ("the young lady from the port town") on business addresses is a nickname for Buenos Aires.