Well that was interesting. Talked to a friend of mine this afternoon about this problem. He used to work at the local farm store and said he used to repair about one bushhog/rotary cutter a week with this problem.
He said a "bushing" cost about $20, cut out the old one (lots of grinders and cut off wheels) and weld the new "bushing" on. Did a search at the Agri-Supply place on "15 spline" and this was one of the first results:
Agri-Supply calls it a "Blade Hub":
Blade Hub for RW61� | Agri Supply #89755
So we're back to a variation that Pmsmechanic mentioned in post #9 above, except with no machine shop work, just grinder/cut off wheel and plasma work.
Comments?