i see two sets of marks....the first is london under 1904-25 rules - arm & sword over NP....nitro proof shot 7/8 oz.... 20 (bore size at 9") on each flat. several additional marks are not visible in your photo, but should be present.
second is b'ham under 1954-89 rules - decimal bore measurements....and 3TONS....crossed swords with dating codes. if you can discern the letters and number amongst the crossed swords, they will tell the reproving date.
i see remnants of choke stamp on left tube....crown R for reproof on each chamber....and at the rear edge of the left flat i think i see LP and a two digit number that would seem to indicate a possible third trip to london??
best regards,
tom