The engine has a Sachs cylinder and piston, The Sachs crank means an early A&M. A&M made the crankcase out of billet.
Later to increase profit they made the MK1 crank trying to copy the Sachs crank. After lots of problems they tried the MKII cantlivered crank which only a little better. They then went bellyup.
Brison bought A&M in a bankruptcy court. Then spent a long time getting the MKII crank right.
AJ sounds like you got a keeper.