So, no I was going by the chart I saw at Sheldon Brown's (RIP) site. This chart makes even more sense as the hubs are a bit more modern than 1970, and the shop it was serviced at (and presumably purchased from) wasn't in business in 1970.

Nottingham, April, 1980, #20,536. Looks like that's my story, and I'm sticking to it.

Thanks, Neal.