This is a restrictor for tilt and turn windows. They break easily when not handled properly, so I decided to make one myself, because I can't be bothered to go to the store and buy one.
(also, it seems by the cost of plastic, it's likely cheaper to print this at home that buy it elsewhere, but that's another story... :) )
Match two printed parts and screw them together with countersunk cap M4 screw. If parts fit too tight together, you may want to sand the tab of the mounting piece to make it thinner. A hole in one of the restrictor tabs intentionally left too tight for M4 screw, so it can act as a self-tapper. After screwing it together, you may want to add a drop of superglue on the end of the screw for taste (or confidence :) ).
And there you have it...