The number 1 in the 51/61/71 series denotes that the caster is a single ball race/kingpinless style swivel caster. These casters are built with a different swivel section than the 50/60/70 series casters. The rigid casters for the 51 and 71 series casters are the same as the rigid 50 and 70 series casters, therefore the rigid part numbers begin with 50 and 70. 60 series rigid and swivel caster rigs hold wheels up to 2″ wide. 61 series swivel caster rigs, the single ball race/kingpinless style caster, hold 2.5-3″ wide wheels. 65 series rigid and swivel caster rigs hold 2.5-3″ wide wheels. Therefore, if you need a rigid caster to match your 61 series swivel caster, you would have to order a 65 series rigid caster.