This is 1943 headstamp from SR Royal Ordnance Factory, Spennymoor, Durham, UNITED KINGDOM. This factory was part of the 1939-45 war emergency expansion plan. It began production of .303 ammunition in 1941 initially with the headstamp code of two arrows replacing these in 1942 with the code SR. The Spennymoor ammunition was filled at the Royal Ordnance Factory, Aycliffe, Durham. Known to have produced .303 cartridges in:
Incendiary B Mk 6, B Mk 6Z, B Mk 7 and B Mk 7Z.
This ammo has a blue tip and is on 5 round stripper clips. Head stamp is SR 43 BviiZ