Jim you have a marking I have found common on some of the Nepal Martini's. 2 M.F. (Munster Fusiliers). There are quite a few post and pre Childers markings I have recorded.
Probably the most common are 2 H.L.I (Highland Light Infantry), S.T.F (South Staffs), S.R (Surreys) and O.X. Oxfords.
If you go to my Facebook page, The Martini Henry Society, and go to "files", I have posted excel files of all the pre 1881 Regimental Codes, with their post Childers codes i.e. 24th >> S.W.B, likewise Ive done the same on the pre 1901 Volunteer Yeomanry and Artillery district markings.
The current go to book is The Broad Arrow by Ian Skennerton, however most of the codings are post 1901 and often confuse matters so in my upcoming book due very shortly I do give a comprehensive Martini only related markings section.