A letter on a gun of that grade is certainly a wise investment, but don't be disappointed if it makes no mention of the second set of tubes. The pre-13 records were not as detailed as the later records. For instance, I know of a 20 Monogram, vintage 1912, that has a second set of Crown tubes, numbered to the gun, that the records makes no mention of the Crown barrels. From what I'm told an owner would order tubes for a lower grade gun to save money. The factory would number them to the owner's frame when fitting them and not record such in the records.