Does anyone know what the initials "BT" mean at the bottom of an AIR 78 card? I know "F" is F reserve.
These two alpha group codes are usually on post war cards. I was told they were cyclic redundancy checks (CRC) for the transcription of service numbers when they files were punch card/computer indexed. Operated like checksums in data packets for transmission today. Ross
The index cards were an updated record as actions were done on the registry BT was added later in a similar way to the WAAF maiden name cards that were annotated with the married surname.