It isn't a "thing", that you buy & install; it's more a gunsmithing "fix" that you just do. However, it requires a certain amount of machining capability/knowhow. It's described pretty well in the topic "Browning 1885 Disassembly", down the current list maybe 10-12 items. It's otherwise known as the "Dale53 fix" and I think you should be able to do a search under that and find the original description.