George,
Lancasters oval bore rifling predates breechloaders and was used initially on artillery, by the 1850's he had experimentally rifled the British Military Enfield .577 cal muzzleloader, some of which were taken into service. Oval bores also made good scores in long range target matches.
I have fired one of Lancasters four barrelled 'Pepperboxes' chambered for the .455" British service pistol round, it was quite a hefty beast but recoil was minimal, it was quick to reload as well. Many people without sufficient knowledge believe that some of these weapons are in fact shotguns, nothing could be further from the truth. Quite a mechanical genious was Mr. Lancaster
Your double rifle could have been made at any time in the last 35 or so years of the 19th.C. The great advantage of the oval bore over any other form of rifling of course, was it was so easy to clean.
The value of your rifle does depend upon condition, but they are a scarce item and not a lot were made. I've only seen two in my 50+ years of shooting and both were in excellent condition.
As with most things, it's only worth what someone is willing to pay. It may be worth your while checking some of the Auction Sites to see what pricess are being made by similar rifles. Lancaster was a top class maker.
Harry
