Xausa, both lancaster and Fuhrmann are right. On the Schmidt & Habermann Mod.21 there is a latch built into the cocking piece knob used as a half-cock safety. here are the parts of such a latch:

This is such an action on full cock:

Here it is with the firing pin all the way forward:

To put the action on "safe" you grab the knob with your left hand, pressing the rear end of the latch in. Pull the trigger and let the firing pin slide forward until the swung out front of the latch rests against the bolt sleeve. The rifle is now safe, with the firing pin off the primer:

To put the rifle on "fire" again, either pull the cocking piece knob back to full cock again, or lift and lower the bolt handle.