From Brass Goggles

A type of bludgeon made of lead or lead shot in a leather casing, used to incapacitate (by clubbing to unconsciousness) or kill targets; legendarily often carried by sergeants in teh British Military up through the 1900s, and used to punish and/or subdue recalcitrant subordinates, especially unruly recruits and "troublesome" individuals in the ranks.

To be fair, it was probably not as widely used as rumor and literary license has made it seem.

Some types were also called "saps."

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