The UIN relates generally to the unit - as in Unit Identity Number as in A01234-A as CM says above but not to the L42. I have never heard of a rifle as being a B item because many stores items are classed as B items in accordance to their Stores Identifiers. Small arms were generally known as WOCS and later as SSR and always marked on the big green air movement labels as a big star indicating a 'STARRED ITEM' which any ex crunchie will have nightmares about loosing! I can't think of an answer to the 5/7 unless there was a 5th Brigade somewhere......... I can't recall one!