Rear sight markings are Proof, High Velocity, Small Cone.
Proof: it was oin th rifl when it was tstd.
High Velocity: sight bd is ground to sight corrctly with 174-grin bullet t 2440 ft/sc MV.
Small Cone: has a normal chmber, not one of the grossly oversized ones they turned out when all the bad ammo was in circulation.
Second photo:
Serial Number
Proof marks, military: crossed flags and P
Commercial proof marks from sale: .303", 2.222", 18.5 TONS/" square (service load, case length, pressure and bore)
Broad Arrow: Imperial Government ownership
Barrel date: '18 is 1918
Hope this helps.