While not a firearms item per se, as these pictures were found in a c. 1910 British Jefferey's guns and rifle catalog, we felt it was fair game (no pun intended).
Ferrets have been used for catching rats and hunting small game, particularly rabbits, for centuries. In Britain, especially, it had and has quite a vogue. Ferreting was so popular, in fact, that poaching with was rife. The poachers would often tie up their cuffs, hide their ferrets in their trousers (don't even want to think about it) and would surreptitiously release them when the laird's gamekeeper wasn't looking.
As the little guys could get a bit rambunctious, a ferret muzzle was just the ticket for keeping a snarly weasel in check. They could be attached with only one hand, leaving the other free, presumably, to wrestle with the less-than-happy wearer. Muzzles were sold in three sizes at the bargain price of nine shillings per dozen, plated extra--such a deal.