David,quite possibly original from H&H as I know they would make non std length chambers per clients request. I had a Royal through the shop with 12b-2&5/8th chambers @ 1900 manufacture, extra bbls made up in 1923 also in 2&5/8th all so marked with London proof.Checked out by H&H NYC and all very original.
Gun was ordered by a wealthy Canadian from Winnipeg, I speculate he had as much opportunity for ducks as upland birds so went for something that would handle a heavier load.
All the best,

Hugh Lomas,
H.G.Lomas Gunmakers Inc.
920 876 3745