If you do some digging around searching for Army &Navy CSL sales records, you can find the source, who will send you an electronic copy (essentially, just an email with the info) for free, or, for a price, copy the page of the sales record of your gun and mail it to you.
A&N CSL was a Co-op that served to sell good to very good quality items, to the far-flung corners of the British Empire, often, but not always, with an emphasis on supplies a serving Gentleman on a military Campaign might need. They tended to serve a bit higher Class of customer than the average.
A decently in focus picture of the proof marks on the underside of the Barrel lump will go a long ways to dating it.
Condition, both from a cosmetic, as well as purely mechanical perspective, is going to drive price.