Hello. Yes this is a spare parts rear sight for a Long Branch No.4 T definately. Probably a spare as it appears unused and never fitted.
Unfortunately for you every Long Branch sniper I have ever seen is complete. Notice that the standard battle aperature is finely milled off and LB blued overtop. The Maltese Cross is an exclusive stamp of the LB sniper rifle Dept. of the plant and it does appear on several
other parts of the sniper rifle and indicates a passable inspection for the designed rifle. Don't let anyone convince you otherwise. JOHN