The 1910-1911 Jeffery,s catalogue illustrates your gun.It is described as a "Long Range" Hammerless, Pigeon and Wildfowl gun.The price is listed at 12 pounds 12 shillings.By comparison in the same catalogue, Jefferies best gun is listed at 55 pounds.
The catalogue states that the barrels are made from "Herakles"
Compound gun barrel steel.Your gun appears to be proof tested to 1 1/2 ozs, shot. According to the specs;in the catalogue your gun should weigh in at between 8-9 pounds.
The gun illustrated in the catalogue is identical to yours. I am unable to help with the date of manufacture beyond that provide by others based on the proof marks.

Roy Hebbes