It's a original glass shelf joint of the ABS plastic, of two types joints (see pictures) copy, that breaks very often when shelf is assembled or disassembled not so carefully. So I grabed a calliper and measured dimmensions and draw in Freecad. More fotos and 3rd type joint model later.
P.S. As I've promised, found last part, and drew it in Freecad. And I've printed, took about 1 hour to print.
PLA, print as ussally 225 C. 100 % for strength.