That device is very interesting and appears to be meant to hold the bolt closed when the rifle is cocked and the safety off.
Which can be a help when stalking close to game. American gunsmith Barney Worthen built sporterized 1903 Springfield rifles in the early 1900 and he added a spring loaded detent in the front of the rear safety lug that snapped into the back of the extractor. It did the same thing.
And a few of the last Mauser sporters had a detent powered by the trigger spring that protruded through the bottom of the action and fit a notch in the rear safety lug that also held the bolt closed