Thank you Roger.
A 50mm Big Apple would fit. But the guy got back to me and said he'd been wrong about the fork and it's the Raleigh type after all. Dang!
I just checked a 50 x 559 and a 50 x 305 and there is a lot more difference, 4mm, than can be accounted for by the difference in rim width. The 305, on an Alex X-101 rim would give me about 1mm clearance with the fork I have. (I build my wheels to +/- 0.004" and they stay true. so this should be enough) I just need to find out if the 50 x 406 cross section is like the 305 or the 559. All my 406s are 60mm.
As for dates, I've always just gone by the hub.