There may be a calibre .300" or .250"stamped along the proof mark.
In Colin Greenwoods book The Clasic Rook and Rabbit rifle he examines a Gibbs rifle serial number B4651 calibre 297/250 . on the underside of the barrel is serial number 69972 and this repeated on the underside of the action body, on top of the barrel has the Bristol and london address and Gibbs serial number B4651
He goes on to state that it was made after 1907 because of the London address and that it was made by Webley and retailed by Gibbs.
Elsewhere in the book he states that when centre fire rook rifle ammunition became hard to get and the licenceing system imposed by the police in 1920 sleeving rook rifles with a 22 parker sleeve was the solution

My own Army Navy side lever .300" rook (Webley) the barrel was pitted and rusted.I had it recut and chambered to 32/20.I only load 5 grains of ADI ap70 for the Big Game Rifle rook rifle event.I keep the load light.The firing pin hole in the action is large and a hot load could cause primer problems
I hope this helps

good shooting

sbs470