I found a note in an old HOLLANDER:

Forged crankshaft used up to engine # HM340-3911-8000

H = 1972 model year
M = Mound Rd. engine assembly plant
340 = 340 cu.in.
3911 = engine assembly date - Wednesday 4/12/72
8000 = engine unit number

much later in the model year than I had thought