Your gun is illustrated in the,"Jeffery,s Guns and Rifles catalogue, issued for the 1910-11 season".The gun is described Jeffery,s No 1A,London pattern 1905 model "farmers"bar lock hammer gun. The price at the time was 5 pounds, stirling. By comparison their best sporting and pigeon hammer gun sold for 12pounds 12 shillings stirling.[ ie: 12 guinea,s]
Looks like a good example. If in proof, with acceptable barrel wall thickness and otherwise in sound condition, one could expect to pay 1000-1250 pounds for this gun in the U.K.

Roy Hebbes