you were right they did do a 305cc
1st 250 1964, 1st 305 1968, 1st 315 1969, here is the best bit most parts interchangable and flow on to around 1976
didn't know shit this time last year its amazing how they have come on line just lately, the irony is the old Norton slayers are mostly being revived in the UK, Browny ex Yamaha Pitmans sold 3 x engines to the poms recently and lots of parts now available in the U.S.A (bugger all 9 months ago) they just recently started supplying Wiseco pistons for the 62mm, 62.5mm and 63mm bores so the old 350 barrels can live again and of course they just go straight on to the 250