This rifle shows no less than four Brenneke "improvements". I have seen all of them before on "Original Wilhelm Brenneke, Leipzig" rifles. BTW most of these rifles have the tiny ESHA stamp of the real maker who made them for Brenneke, Schmidt & Habermann, Suhl, in a hidden place. But some of the "Brenneke improvements" were actually designed by Albert Oskar Merkel, Judithstr. 60, Suhl. Oskar Merkel left his brothers Gebrueder Merkel company in 1908 to set up his own gunmaking company. Note his Warenzeichen = trademark "MERCO in shield" in one of the drawings.
Brenneke had the thumbhole in the left receiver wall welded shut, way before Mauser made sporters with solid receiver walls.
The small box, fastened with two srews to the right side of the bridge, contains a spring and plunger that engage the bolt handle to keep it from being raised inadvertedly.

The Oskar Merkel designs shown in the drawings are
An override trigger assembly, similar in function to later American designs like the Timney. This markedly improves the pull of the unset double set trigger when used in an emergency or on running game. In the drawing it and it's parts are shown with a single, shotgun style trigger blade hinged in the triggerguard, but on the rifle offered it is combined with an additional double set trigger.

Oskar Merkel also designed that unusual floorplate latch with that giant thumbpiece, as shown on rifle and in drawing.
IMHO all, maybe except the override trigger, rather doubtful "improvements" of the basic Mauser M98 action.