Bridgeport J head step pulley "extra loose part"

I picked up my mill today, and when I was turning the head over to load it I noticed a spacer about 3/4" diameter with a 3/8" bore about 1/2" thick. Looks like a hardened spacer, I have not found the exploded view to see where it may have come from. Any ideas?

Thanks Guys!!

I got a Kurt D866 on the way at least!