A headstamp is the marking on the bottom of a cartridge case designed for a firearm. It usually tells who manufactured the case. Military headstamps usually have only the year of manufacture .
A letter, number, or alphanumeric code indicates the place of manufacture: Other codes may indicate metal suppliers (like France or The Netherlands) or the percentage of copper in the alloy the cartridge case is made of (like Poland and Germany).
United States military cartridges
US arsenals
World War II manufactured cartridges with a single "4" as the year of production were made in 1944. The demand was so great for cartridges that most manufacturers just ground off the "3" on the "43" stamp to save time. Most wartime plants were closed by the end of the war. Some cartridges with a single "5" were made in 1955.
AO or KS Allegheny Ordnance Plant (Kelly Springfield) (1943–1945) – Cumberland, Maryland; Operated by Kelly-Springfield Tire Co., a division of Goodyear Rubber. Conversion back to rubber tire production began in 1944. CB Curtis Bay Ordnance Depot (CBOD) (1917–1957) – Baltimore, Maryland. Used to store, examine, overhaul, and repack ammunition. DEN Denver Ordnance Plant (1941–1945) – Denver, Colorado: a division of Remington Arms. DM Iowa Army Ammunition Plant (January 1942 to July 1945) – Des Moines, Iowa: a division of US Rubber Co. EC Evansville Ordnance Plant (Chrysler) (June 1942 to April 1944) – Evansville, Indiana: a division of Chrysler-Plymouth. They manufactured brass-cased ammunition. From 1943 to 1944 it manufactured steel-cased .45 ACP ammunition. ECS Evansville Ordnance Plant (Chrysler-Sunbeam) (1942–1944) – Evansville, Indiana: a division of Sunbeam Electric Manufacturing Company. Chrysler bought the plant to keep up with demand. It loaded cartridges made at the Evansville-Chrysler plant across town and then packed them in shipping crates for shipment. In November, 1943 it was the first plant to package ammo in vacuum-packed metal cans. In the Spring and Summer of 1944 it was employed in inspecting and repacking .45 ACP and .30 Carbine ammunition. EW Eau Claire Ordnance Plant (August 1942 to December 1943) – Eau Claire, Wisconsin a division of US Rubber Co. FA Frankford Arsenal (1816–1977) – Philadelphia . HAW Hawthorne Naval Ammunition Depot (1926–1977) – Hawthorne, Nevada, USA. HW- Hawthorne Army Ammunition Plant (1977–present) – Hawthorne Nevada, USA. From 1977 to 1992 it was designated the Hawthorne Army Ammunition Plant (HWAAP) and from 1992–present it was redesignated the Hawthorne Army Ammunition Depot (HWAD). KOP Kingsbury Ordnance Plant (1941–1945; 1950–1959) – Kingsbury, Indiana. LC Lake City Ordnance Plant (operated by Winchester Arms) (1941–1945; 1951-Present) – Independence, Missouri; A sub-contractor originally owned by Remington Arms. It is currently owned by the US Government and operated by Northrop Grumman Innovative Systems. LM Lowell Ordnance Plant (1942–1943) – Lowell, Massachusetts LOD Letterkenny Ordnance Depot (1942–present) – Letterkenny, Franklin County, Pennsylvania. It was renamed Letterkenny Army Depot in August 1962. LOP Louisiana Army Ammunition Plant (1942–1996) – Doyline, Webster Parish, Louisiana, USA. LS Lone Star Army Ammunition Plant (1941–2009) – Texarkana, Texas, USA. M Milwaukee Ordnance Plant (August 1942 to December 1943) – Milwaukee, Wisconsin: Operated by US Rubber Co. P, PC, PCC Kings Mills Ordnance Plant (Peters Cartridge Company) (1942–1944) – Kings Mills, Ohio: A sub-contractor owned by Remington Arms from 1934 to March, 1944. It was converted to only produce .30 Carbine ammunition in 1943 and produced more than Lake City by 1944. It was closed down in March, 1944 to consolidate production at more centrally-located plants. The site was converted to press vinyl 78rpm records for Columbia Records from 1944 to 1949. It was then used for storage from 1949 to 1968. PBA Pine Bluff Arsenal (1941–present) – Pine Bluff, Arkansas, USA. ROP Redstone Ordnance Plant – Huntsville, Alabama (1941–1949). Combined with the Huntsville Depot facility to form Redstone Arsenal in 1949. SL St. Louis Ordnance Plant (November 1941 to June 1945) – St. Louis, Missouri. SND Seneca Ordnance Depot (1941 - 1990s) - Seneca, NY. TW Twin Cities Ordnance Plant (1942–1945; 1950–1957; 1965–1976; 2002–2005) – Minneapolis. U or UT Utah Ordnance Plant (March 1942 to December 1943) – Salt Lake City; a division of Remington Arms.