One other thought:
You mention that the rifle is currently chambered for the .577 3” cartridge.
There is a distinct possibility that it was originally chambered for one of the older & shorter versions of .577’s (Snider, 2 1/4”, 2 1/2”, 2 3/4”j and due to ammunition obsolescence and availability had a .577 3” re chamber job to keep it shooting.
Yes, it “should have” been re proofed but if the job was not done in England then likely no re proof marks.
Just more 2¢ worth.
- Mike