If it is actually a 12ga it would be less common than having 16ga barrels. I don't believe that the barrel selector on that model cocks the rifle barrel, it only selects the barrel, unless there is a larger knob behind the selector shown.
Caliber is most likely 8x57R shooting .318 bullets. Most of the .323 diameter 8x57R are in post war drillings.
It looks like the date might be 11/35 (Nov. 1935).
Look for a 12, 12/70, 12/65, 16, 16/65 or 16/70 I don't know the dates when they started designating chamber length. If there is only a 12 o 16 in a circle or diamond they are 65mm (2 1/2") chambers. It should be on the barrel flats which aren't shown.
I admit I'm not an authority on drillings but a love to hunt with them and have three and am seriously looking at number four if I can get the gun fund back up where it is needed.
Welcome to the addictive world of drillings.
