I bet that is a removable mobo tray. On the back end there are tabs for either an individual back panel to slide on and be secured or to slide into a part of the back of the case itself with non-removable back panel. It also has tabs on the front side for screws to secure it in the case.Is that a "removable" motherboard tray, or did you cut it out of a case? It's certainly something that a lot of our members might be interested in trying.
I bet that is homemade as it lacks screw holes for each slot. And notice how he bolted the cards in, the holes are not tapped. (I would have just used a screw and a nut as well, I do not like tapping holes much)I'm also curious about the card rack you've got there.... did you manufacture that yourself, or is it also part of a case.