According to Schwiezter's book, the addition of the 50mm to the swingarm tube was:
R50/5 - from 2906304 which is the start of the 12/72 build.
R60/5 - from 2947966 which is the start of the 12/72 build.
R75/5 - from 2997998 which is a few bikes after the start of the 12/72 build.
Kurt in S.A.