There are 5 different H2 axles and 3 different styles the easiest way to tell is the end opposite the thread : 
1971 - 1972 H2 completely round 
1972  Round with a hole  283mm
1973 Round with a hole and 2 flats and 4 mm longer (?) 
1974 Large round with no hole and 2 flats and 6mm Longer 
1975 Large round with a hole and 2 flats 8mm longer and also uses a washer behind the castle nut.
H2/A swingarm bolt has large squared off head. H2B/C swingarm bolt has a shorter domed head.

KH (top) vs Late H1 Front Axle
(not interchangeable)