FWIW, the cutoff was first omitted in 1915 by BSA, prior to even receiving approval from the Ministry of war, for which their fingers were soundly slapped. By 1916, all other makers except LSA (changed over in 1918) had also omitted it along with adjustable rear sights, volley sights, piling swivels, milled rear sight guards and the stock marking disc. some makers re-introduced it on new production in 1919 onward.
In 1919, a good many MkIII* rifles had the cutoff retroactively milled into the receiver and the cutoff fitted. the * was then lined out on those rifles. This explains why we sometimes see the ubiquitous 1917 BSA's with cutoffs, or NRF and SSA rifles with cutoffs, though those makers never amde a rifle with a slot as new production. None of the other MkIII (no star) features were reinstated though and the sealed MkIII pattern was altered to be a MkIII* with a cutoff. Many MkIII's then in service simultaneously had the adjustable sights pinned so they could no longer be adjusted - all rifles must operate uniformly for training consistency, I suppose. Some time in the late 1920's (1929 rings a bell but I don't have skennerton with me), the UK factories stopped fitting cutoffs altogether. Lithgow and Ishapore, on the other hand, stopped fitting it in the late 30's. The odd 1940's Lithy or Ishy are found with cutoff slots, but generally those cutoffs were never fitted in service and the are thought to represent earlier 1930's receivers that for some reason were not assembled until much later.