Similar to the English Sporting Rifle plug and tang, this smaller tang only is made for a 15/16" octagon breech. Sometimes called a "rook rifle" plug, the English made small bore lightweight sporting rifles for shooting pesky birds. It has a longer tang, cast of malleable steel which may be bent cold.
This is the tang only, no breech plug for this tang is available. The tang is 3-3/4" long.