I knew some "Williams", but over there, you can't throw a dead cat without hitting a "Williams", if I remember right. I was attending WIU to get my Law Adminstration degree back then (after I'd come from Bradley in Peoria, Electronics Engineering). Finished in March '74.
We had a 16-year old kid at the shop named Foxton or Foxall or something like that. He started out changing tires and fixing flats, a skinny kid. Then one day, he was riding a customer's bike on a test ride after the flat repair and crashed, without his helmet, into a car. I jumped into his Mustang in the lot and went and got him, in deep shock, and took him to the hospital. He "lost" about 3 days of memory after that and was off work for 2 weeks before he came back and quit to take some long time off. Nice kid. That was in the summer of '74, right when I left for Colorado. I hope he was OK.
Bearings: I just came across a KML bearing number that may help us all out. It's #32006X, with ISO 355 class number T4CC030. This is a 30mm ID bearing (just right for the lower end of the triple tree), but the cup is 55mm OD. If we can find a 52mm OD or 50.005mm OD cup with the same taper, we're home free. Grinding off 5mm from the 55 will leave insufficient raceway, but removing 2mm from a 52 is no big deal. Can you find a "cross" for this type?
The 07100 series has the 50.005mm cup, but the taper is 3 degrees different, like the crummy boxed Japanese sets you can get. This makes a poor long-term match.