I started working on "A"s in 68 Naha Okinawa 55-56-57 year serial #s ( 815th, 41st, 35th, and 21st) which had 3 bladed props which were converted to 4 blades in late 70's The "B"s are 4 bladed but no external tanks between 1-2 & 3-4 engines. Worked them in country with the boys from Clark in the P.I. In 85 I joined a reserve unit in P.A. Willow Grove 913th A/W 327th flying Squadron we had "E"s built in 63, they had external tanks between 1-2 & 3-4, the "E"s had HF antennas running from top of fuselage just above crew compartment to below top of rudder. Although some early "H"s had the antenna but they had T-56-15s (Dyess AFB) compared to the "E"s which had T56-7s. Worked on newer "H"s at C-Springs, Pittsburgh, Niagara, Maxwell, Dobbins etc etc and the best way I could tell difference was no wired HF antenna running length of upper fuselage. And od course they were newer looking. But as a Pro Super late in my career I always just checked the forms! Al Udeid 05-06 we had 63 model "E"S and "H"s built in the late 80s early 90s retired Pope 09 we had 87-88 model "H"s.