Huvius - Well spotted! I did not think about Hollis & Sheath but it looks logical although Brown's Gunmakers does not give a clear date when I Hollis & Sons took over from Hollis & Sheath

The last date of Hollis & Sheath appears to be 1861 in Birmingham, when I Hollis & Sons took over. Their London premises opened in 1870

This fine rifle could be either side of that date? (1861)

My Snider is later than that, certainly closer to 1870, and there is a Martini posted on British Militaria Forums with the stamp which must be later again

The probability is that I Hollis continued to use the earlier trademark for several years, especially with the "Makers to Her Majesty's War Department" on it, maybe as an extra sales pitch
What do you think?

Tony