What you have is, effectively then, a N/No9 rifle, but probably converted by a civvy gunsmith here in the UK to that spec. Interestingly, back in the 90's I bought from a school five such converted No4's. Barrels were just as yours, & they bore no official N/No9 nomenclature. Neither were the muzzles crowned & Parker-Rifled stamped. Very much like yours indeed, apart from the fact that the butt sockets on mine had been scrubbed, & they had all been renumbered, presumably when converted. No odd hieroglyphics on the receiver rings either though, I'm afraid, so if your hypothesis is correct, probably not the same gunsmith!
Re the US flaming bomb stamping; are you making the point that it is unusual by being stamped on the rifle at all, or just where exactly on the rifle it is located? I have always thought it was, broadly speaking, a US equivalent to the Enfield examiner's mark, indicating acceptance/ownership by said government. The Savage No4's were owned by the US but then provided to
Britain & Commonwealth forces, but IIRC the US retained title to them. I just went into the gun room & checked three Savage No4 T's. All of them bear the flaming bomb, though all are marked on the butt socket, not on the body side wall. So, what I'm saying, I suppose, is that I don't think bearing the flaming bomb is anything unusual, unless you feel it may be because of where on the rifle it is marked. I could well be wrong on this as I don't see a lot of non-sniper No4's, so shoot me down in flames if I am.
I don't know if this helps any, but it's nice way of spending a Sunday afternoon! And don't worry, I agree totally with your comments of 'never say always & never say never' with Lee Enfields. I simply meant that I've not seen heard of or owned a Savage rifle that left the factory without the US Property stamp.....to date.
ATB