By Scott Duff
Another factor (in addition to drawing numbers) used in the identification of Springfield Armory manufactured receivers, barrels, and bolts is the "heat lot" number. This is an alphanumeric code used by the Armory to identify the discrete lot of steel from which these three critical components were manufactured.
Springfield Armory stamped the heat lot of the batch of steel used in the manufacture these three components in a location near the drawing number. The identification of the lot of steel used in manufacture of a group of components could be very important from a manufacturing perspective. Should a metallurgical problem occur relative to a particular batch of steel, all components produced from that batch could be identified and recalled. An example of this may be found in TB ORD (Technical Bulletin - Ordnance) 366 which instructs that all bolts marked with heat lot O-16 be scrapped. The reason is not stated but is thought to be related to the metallurgical properties of that heat lot.
An accurate data base of heat lots relative to serial numbers permits collectors to identify a receiver which may have been assembled from demilitarized receiver halves, verify the correct bolt for a specific time frame, and identify barrels which may be the product of unscrupulous individuals.
Excerpted from The M1 Garand: World War II copyright 1993 by Scott A. Duff