The image unfortunately is low resolution and very small

[Linked Image from photos.smugmug.com]

The original proofs were London, and the second Birmingham proofs are clearly stamped over the 'Siemen Steel'. I can't date the London Proof
The Crowned BNP (Birmingham Nitro Proof), and the .729" are 54' - 89' and the mark in the lower L is the Birmingham date code used 50' - 74'
If you'll tell us the letters within the halberds I can give you the date.

3 1/4 Tons is for a max. service load of < 10,000 psi

BTW: it those aren't enough proof marks, you can buy your own!
https://waffenamt-shop.com/gb/99-british-proof-mark-stamps

https://sites.google.com/a/damascusknowledge.com/www/home