There really is very little differance in any of the Hs. Pretty much an H is an H, irregardless of the year it was made. The exceptions would be the 1939 and early 1940 model years. Those earlier tractors did have some very minor differences such as the seats and I think there was some differances in the belt pulley drives as well. Also, there are some differances in the internals of the transmission and rear ends which might cause some interchangeability issues with later tractors when hunting down parts. One of the differances I have encountered just recently is the bull pinion shaft and brake drums. It seems that the Hs built prior to serial number 70,287 have a different bull pinion shaft, and the brake drums used on those models will not interchange with the pinion shaft and brake drums used on the later tractors, but the brake bands themselves are fully interchangeable.
In short, to ensure you do not run into any of these issues, just look for an H with a serial number of 71,000 or higher.