With some minor changes it became the basis of the 1962 United States Tri-Service aircraft designation system.
This information, along with the name of the service (USAAC, USAAF, USAF), the base (if permanently assigned) and the serial number was painted on the forward fuselage side under the cockpit.
Minor modifications to a basic design are usually given a sequentially assigned letter denoting the particular subtype, starting with A and continuing with B, C, D, etc.
Analogous to the order number, these help not just to identify when an airframe was built, but in some types distinguish changes that occurred during production not identified by the sub-type letter, as between the Republic P-47 Thunderbolt razorback variant and the bubble canopy variant.
Each factory producing aircraft for the USAAF was assigned a two letter code to distinguish between the product of one facility from another.