The stamps are the "proof" marks. We have a quaint custom here in the UK and Europe that gunmakers have to send their guns out for independent testing at a proof house before sale to the public to make sure they won't blow up. The proof house then puts their stamp on the rifle or shotgun. Your Westley will have been proofed at the Birmingham or London Proof Houses. You can look them up on the web if you are interested in your rifle's history.

doubleriflejack basically explained what the marks mean. There is more in Graeme Wright's book, now into its 3rd edition.

Westleys have or at least had an operation in the USA - you could do worse than drop the rifle off with them to investigate. I hope the rifle has not been converted to the "improved" chambering. The 318 was not meant to be a rocket like the 270 Winchester let alone your US magnums. Boddington considered it a fine enough cartridge in its plain jane formulation.