The British military started using training rifles in 1883, with the .297/.230 Morris cartridge in adapted Martini rifles. This would give way to the .22 rimfire cartridge for training shortly after the Boer War, and a substantial variety of rifles converted to .22 rimfire. Standardization would take until 1921, when the “Rifle, short, .22 inch, RF, Mk IV” was formally adopted – a conversion of the No1 MkIII SMLE to a single shot .22 rimfire weapon. This was modified to Mk IV* in 1925, when an empty magazine body was added to the rifle, to act as a brass catcher.
Just to make things more confusing, the nomenclature system was retroactively changed in 1926, and the designation because Rifle, No2 Mk IV*. This rifle is a very simply conversion. It used a standard bolt body, with the striker and bolt head modified for a rimfire type firing pin and .22 caliber extractor. The sight was not even changed; instead a conversion chart was issued with the rifles to specify the proper sight settings for .22 rimfire shooting (ie, set sight to 300yd for shooting at 25yd). These rifles would be used into the 1950s, particularly by India and Australia, who did not produce No4 rifles and thus did not produce No4 trainer conversions either.