After 8 weeks I remembered to telephone John Dickson & Sons who hold the records for Alexander Henry
They were very helpful, and 10 minutes later had the copy of the original ledger entry

The result was much better than I had expected

No. 4048 - express rifle, best quality! 28" octagonal barrel, left hand bolted lock, bolt on guard, sold 7 January 1877
to the Duke of Marlborough!

This was John Winston Spencer-Churchill, the 7th Duke of Marlborough, the paternal grandfather of Winston Churchill!

This explains the extras and less usual octagonal barrel and very unusual lock on the operating lever

Daryl - I looked up the .500/450 No. 2 musket, the top load is a 540 grain bullet ahead of 90 grains of black, it should be good for your mooses? In another world I would love you to take it and use it for that

I am most pleased and happy to share with fellow enthusiasts

TH44