A) Your late fathers gun is a Midland Gun Company Magnum Knockabout shotgun. It was a heavier version of the standard Knockabout gun, built with a weightier breech and designed to handle 3in cartridges. The barrel also had a raised rib and the company advertised it as a pigeon, duck, wildfowl or trap gun. It cost £4. 10s (£4.50) or £5 for the engraved version. The standard barrel was 34in, while 36in barrels like yours were made to order and cost five shillings (25p) more. Rubber recoil pads were 12/6 (63p) extra.
The Midland Gun Companys records no longer exist so I cant date your gun. However, the 1937 Midland catalogue describes it as a new line, so I guess it was introduced in the early 1930s and was probably almost new when your father bought it in 1938. There may be a small Birmingham date mark on the barrel. This is a cross with letters and numbers.
If you send a drawing to me c/o Shooting Times I may be able to give an exact date.