Two things to look at closely with the older English .410's. A great many were made with 2" chambers as the 2 1/2" cartridge didn't really become available until around 1912/13. Some may have had the chambers lengthened without re-proof. Check the Proof marks. If chamber length isn't mentioned then it is a 2". 2" cartridges in Britain are no problem to obtain but may be scarce in the U.S. The other thing is that they were no often cleaned properly in the barrels and would have been made and used in the era of the corrosive primer.
Hard to value without close examination. Double .410's always fetch a premium over the larger guages. Lagopus.....
