Trev, if "2 1/2" is marked on the barrel flats and if your grandfather received the gun in 1919, there's something a bit odd afoot. Brit proof rules did not require that the chamber length be stamped on the gun (as long as it was the standard 2 1/2") until 1925. What you should find is a shot charge (probably 1 1/8 oz). If you find the word "Maximum", that would make the gun even older (1896-1904). But as mentioned above, simply by providing the serial number, you can get more information from Glasgow University. For a fee, you can even get the order book sheet listing your gun, original buyer, date, who made it, etc.