48 should make it a BSA. The frame though may be a lot older, 1930's, than the date on the rack suggests. I've recently passed mine on to a real military vintage cycle enthusiat (Knees are giving up) the frames were manufactured as one model, then fitted according to order. The holes in your frame that hold the back brake rods on the Mk V* are present on a MK V frame, with blanking nuts screwed in. A lot were supplied to US Forces, over there the preference is for Coaster brakes in the rear hub, though BSA were a world leading manufacturer of them. My guess is that a Coaster brake is a complex thing to manufactuer, given war time demands the twin rod system was decided on to save production cost and time. But it does tick all the Military Boxes. Rifle barrel by the way, it should fit onto a loose fitting canvas strap attached to handle bar, though your way is the most common seen. You are missing the rear U shaped rifle clip that fixed to frame, hence carrier is flat is on one side.