I have found reference likely explaining the "S" stamped on SKYBOLT's rifle.

Gun Digest 1977 edition; "A History of Proof Marks - Gun Proof in England", article begins page 161.

Page 165, referencing the Proof Act of 1868; paragraph titled "Special Rules":
"......Another special category covered British Government Arms or those of the East India Company which had been declared surplus and sold. If these bore the letter S, meaning safe or serviceable stamped over.......they were considered legally proved".

So possibly originally an East India Company rifle that was stamped with the S and then resold.
Now according to Encyclopedia Britannica the actual "East India Company" ceased to be independent in 1873, and thereafter lived on as an adjunct to the British Government. Not British Army but maybe originally British civil service related? Only H&H would know.

My iPad and IMGUR are not playing well with the internet right now so I cannot post the pictures of the relevant section of the article.

- Mike