IMHO it's a BSA, although could possibly be a F. BSA most likely. It's NOT a Maltby -- or at least I've never seen one (having seen a few early rifles) with the panel towards the front of the receiver side wall just behind the receiver ring milled out. Amongst UK mfr the milled panel is typical of BSA & Faz.
ATB
P.S. You can tell for certain betwen BSA & Faz by taking the rifle out of its woodwork & looking at the draws. If they are symmetrical (with lightening cuts) then it's a BSA. If the one on the bolt handle side of the rifle is lightened but the one on the safety catch side isn't, then it's a Faz.
