I took comparison photos and measurements today between a May '69 YH wheel, and a Sept 76 FW wheel. Basically, everything rszmjt has previously posted about teh differences is correct, wtih the only thing I noted is that I measured the width of FW slot slightly less than 1" (~ 7/8"). The length of the slots in both wheels is approximately 4" and the distance from slot to slot is approximately 3" in both wheels (no difference). The differences are what he described. 1) 180 deg opposed reading of the stamped codes at the air valve, 2) difference in the narrow dimension of the slots (YH larger), and the different stampings inside the wheel.
It should be far easier for all of us to determine wheel type from now on..