I take it you have seen the date list in Jonathan Kirton's "The British Falling Block Breechloading Rifle" which gives the first barrel number as 5517 and the second (1920) as 6648 (in .256 Gibbs Flanged)
"Sporting Rifle, Style "C" ordered by W.P.H. Jones for George C.L. Dewhurst, out 30 Jun. 1910"

Several of the previous numbered rifles were .450/.400-3 1/4 inch, others .303. I would have thought the .450/.400 would have been more likely but who knows?

Mark Crudgington must be your first contact, he was very helpful for my Gibbs/W.J. Jeffrey rifle

The extra/additional fittings could have been specified by the owner, or added at the discretion of George Gibbs Ltd.

A superb rifle of the best quality, the chequering on the butt screws is the epitome of the detail put in these fine rifles

TH44