This is the prototype REX AKB15 made by AREX, a Slovenian gun and ammunition manufacturer based in Šentjernej. The base rifle is a Yugo M80 which is bolted to a forend/rail and a rear stock adapter. It includes a full length top rail, quad rail on the forend and a side folding stock that resembles an FN SCAR stock. Looks nifty.

What is not clear to me how the receiver cover is removed. Any ideas?

Thanks to Daniel for sending this in.