M939 is the series, just like M809 or M39. The cover of the 24P manual I have says:
TECHNICAL MANUAL
UNIT, DIRECT SUPPORT, AND GENERAL SUPPORT MAINTENANCE
REPAIR PARTS AND SPECIAL TOOLS LIST
FOR
TRUCK, 5-TON, 6X6, M939, M939A1, M939A2
SERIES TRUCKS (DIESEL)
Then, it goes into the individual...