Mike's(BOTR) answer.
Musketry Regulations 1909 §243:
"To unload - ... then close the breech, press the trigger, close the cut-off by placing the right hand over the bolt and pressing the cut-off inwards, apply the safety catch, lower the backsight..."
Stops the bolt being knocked open accidentally.
It's also the same for the P14
It says the same thing in 1924, 1942 and 1955 as well