Doverham, your Gallyon has been through re-proof but it looks as if they missed the 'R' with crown over mark.

English 3" guns used to be proofed for the old 3" case with 1 1/2 ounce of shot then Eley upset the apple cart by bringing out another 3" cartridge loaded with 1 5/8th. ounce which required guns proofed for 4 tons per square inch. Caused a lot of confusion back in the 60's. One thing to watch with the older English 3" chambered guns. They are o.k. with 2 3/4 inch magnums but not more modern 3 " stuff. Lagopus.....