Only the A2s have the 6CTA 8.3 Cummins motors. Ted is right...all M939 series have Allison transmissions and a Cummins motor (the A0 and A1 had the NHC250, while the A2 had the 6CTA). All have air brakes. Only the A1 and A2 have super single tires...although you can convert an A0 to A1 status simply by installing single tires (i.e., that is the only difference of note between the A0 and A1).
Matt, yes, that is the NSN for the M928A2.