.303 = the calibre.
2.22" = length of the cartridge case
18.5 TONS = proof test pressure in long tons per square inch.
NP under arm and scimitar = London Proof House = Nitro Proof
The BSA symbol on the Knox-form is not three SMLEs, but three Martini-Henrys, 'piled', in military parlance, or 'stacked', as it is sometimes referred to. Hence the BSA trademark motto for their airgun pellets - 'Pylarm', a pun on the term.
More, no doubt, from the real experts soon.
tac
BTW, your rifle was sold out of service, hence the civilian-style proof stamps.