I believe the smooth bore versions were offered along side the rifled versions rather than before and were aimed at funfairs etc.
With the exception of a prototype .22 I believe all barrels encountered on both models are going to be .177
Cadets have fixed sights whereas the Majors rear sight is adjustable,often missing and costs the same as the rest of the rifle in some cases.
In both cases there are earlier and later styles of stock. The later has a traditionally recognised stock bolt whereas the earlier has a screw in the rear of the trigger guard.
DCL Dave on here makes both the piston & breech washers for these and very good they are too.
Most spares are available from the usual places but the are pretty much bomb proof anyway.
Atb Phil