If you cannot see a latch then it will undoubtedly be just a straight pull-off. Once you have pulled it off then life can get interesting! You may find some rust under the fore-end due to rain whilst out in the field, but that will just be cosmetic. The 'A&T' marking may well be found, as Adams and Tait made an awful lot of barrels for rook rifles. Also Birmingham proof marks will be there, I guess.

If it is now a .410 shotgun then there will be proof marks from the 1920s period, probably for the 2.5" cartridge. I have one and it shoots quite well as a shotgun.

Once you have found a gunsmith able to re-line the barrel then keep us informed with some photographs of the work. Mount your scope as low as possible! A straight tube of 3/4" diameter would be in keeping with that era of rifle.

HB