One of the rifles I just brought back from the mainland. Research convinces me that this is a #5 MK I, not a cut-down No.4 as is so often seen. It has a couple of peculiarities, including an almost total lack of markings.
Odds are it's a BSA. They were known for light markings, and it does bear a proof stamp from their Birmingham factory. But aside from that proof, the receiver is devoid of markings.
Can't wait to see the reception I get at HPD for a gun that has no manufacturer, no caliber, and no serial number!