Nr. 4 Mk 1 Barrel Markings - Need help

[LA Gman]

Thank you so much. I really appreciate it!
I can't wait to fire it. I had it cleaned and inspected already.

The muzzle markings are UK Civilian Proof markings, the tests were undertaken by the London Proof House

Calibre = 303
Cartridge length 2.22"
Test pressure = 18.5 tons per square inch

Side wall markings

No4 Mk1
ROF(F) = Royal ordnance factory Fazakerley
Date of manufacture = April 1944

Butt Socket marking
Serial number = MB21601