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This is a picture of the chamber area. I would say that you are probably correct with the chamber being bored out. I have looked it over closely and it appears to be a high quality bore job. With that in mind I wonder if it was done at the factory.
Yes, the right is definitely a re-bore. No, it couldn't have been done at the factory, nor is it at all likely that it was done in England, as such work without re-proof has been illegal there since long before this rifle was made.
From the locations on the gun, 973 is Army & Navy's serial number, and 1646 is the trade maker's serial number. From the number, the gun's features, and it's proof marks, I think this is an 1870's gun. It might be possible to confirm all of the above from the A & N records archived in Glasgow, IF records that early survive (the company was founded in 1871), and if their format is the same as it is in the later ledgers. It's worth a try.
Given the conversion, I think the asking price is rather strong. Were the rifle original and in excellent condition, it would be worth several times that.
Hope this helps.
