I bought a couple of complete Martini Henry rear sights off ebay ,they fit the mk3 barrels perfectly ,they have a fixed standing leaf and a ladder thats is graduated to 1000 yds if I recall correctly .Hope fully they are graduated for the 303 cartridge sighting when the converted them to 303 from 557/450
I plan on using them on my lee enfield projects to replace the military rear sights.
I am currently in the process of making a 5 shot magazine from a standard 10 round no 1 MK3 magazine ,the same as the magazine used in the no4 pattern BSA sporting rifle chambered in 375 x 2 1/2" from the 1912 BSA catalogue .Its a simple conversion if you have some metal working skills and some hand tools.BSA did not make a special magazine for this cartridge ,they just used a slightly modified standard 10 round magazine.

Bob.aka Kev