These are 20" WM2 chromed rims - as would have originally been fitted to Rigid and Gardengate Norton International rear wheels. These rims are of good quality English manufacture, from my normal rim supplier and have the correct spoke alignment for both rigid and plunger cotton reel style rear hubs.
Update 12/12/11: I notice that Vintage Tyres now have a new stock of Avon SM 3.50 x 20 Rear tyres, which are perfect for this rim - I have bought one myself, the link is: http://www.vintagetyres.com/display.asp?K=20111215_3176_000619&m=74&ds=motorcycle%20tyres&dc=101&sort=sort_size&mw=2&st_01=motorcycle&sf_01=type&sp_01=not&st_02=tube&sf_02=type