Nortons pre Commando all use narrow rear chains: 5/8"x 1/4". But sometimes you'll find drums for wide chain 5/8"x 3/8" . The later AJS and Matchless (60's) use Norton wheels with a wide chain. Commando's had wide chains. So please check before you order.
Wide is about 9mm, narrow about 6mm on the sprocket teeth.
Thread on the studs is usually 3/8CEI (26TPI), but Commando drums have UNF (24TPI) thread.