Why the musket cartridge? I know the Isle of Man is the most independent of the five nations I can see from near my home, and not less content than the others, although your Queen has another job. But it is likely that a lot of firearms have arrived there since 1937, when new legislation in the UK meant that a lot of rifles were converted into shotguns, some badly but some very well. There is every chance that it was a 20ga. although the rare 24ga (which I have for a 24ga shotgun) is a close fit in the .577 Snider chamber and often used as such. 2½in. was the usual length for the 20ga chamber in those days, and 28ga always a relative rarity in the British Isles.
It is indeed badly rusted. The classic tool for measuring how deep those pits go on the action sides is a point micrometer. It might or mightn't be possible to remove them (belt-sanding being the best method), and originality of finish is no great issue with this one.
We live in the Information Age, and quite a range of Sharps parts, including a reproduction 1863 percussion hammer, are available from
http://www.ssfirearms.com/search.asp...&sprice=&pg=10 . I don't know the position on export now, although they sold me some straight-pull Winchester-Lee parts a while back. Similarly you can see a good range of rifled bore liners on
www.trackofthewolf.com , but they no longer export them, although they are uncontrolled in the UK, and they sold me a .32 rimfire Webley without the slightest problem at either end. Think of the hammering world terrorism is taking from all that!