ConwayB (or anyone else who might know...),
The letter to the immediate left of the dash is the BASIC MISSION symbol. This includes C for Transport (Cargo), F for Fighter and B for Bomber, etc.
Something that I've pondered in the past briefly but does anyone know why WWII-era Fighters used a 'P' prefix (whilst the Bombers were already using 'B')? eg P-51, P-47 etc vs B-17, B-24 etc.
B73 (A very late model 'Bomber', it would appear...!)
PS Apologies for slight thread creep...