Stroked version of the 600, improved (as in less self-destructive) in 1960 with new cylinder head and lubrication system; plus duplex frame with better steering, though it wasn’t until ’63 that they had the Roadholder forks (and 12V electrics). Lots of early bikes retro-fitted with later equipment. CSR version had 45hp engine that could be a touch fragile. Beware, again, of bodged engines with non-standard parts. Also Matchless G12.
1958-66 646cc (72x29mm) OHV twin 40hp, 105mph, 55mpg, 400lbs