That's a '52. What year were they introduced? Maybe rare.
Pivot bolts are different than the other 50's GS rear on ebay as well. Maybe another clue.
Edit: According to VeloBase the '52 was the first year and the only year to use those pivots.
ReEdit: defer to CdM post #10
Last edited by Ex Pres; 11-26-10 at 09:09 AM.