Body
Types could vary. Pictured is a brass knuckleduster with four finger holes. They were smooth knuckles and had a bar which rests against the palm.
Another is a BC41 knuckle duster. The BC remains a mystery but some sources quote it as meaning Bliss Casters or Birmingham Casters with 41 being the date. It is made of cast iron.
Be aware Knuckledusters marked BC42 (in emulation of the BC 41 Knuckleduster-knife) are a known pattern of fake.
Size 18cm x 12cm
References
John Pidgeon