This is a Y axis bearing block which actually work with the old Prusa i3 Acrylic. I purchased my Prusa i3 Acrylic and the instructions always required you to use some zipties in order to attach the bearings directly to the acrylic Y axis bracket. This bracket replaces the need and allows you to properly mount the bearing to it.
This model requires 2x 15mm M3 screws (However, 20mm screws should work fine as well) and 2x M3 nuts. You will also need two zipties, the smaller the better.
In order to assemble it, all you need to do is push the nut into the holes on the sides, until you can see through the screw hole properly, then insert some zipties and attach it with the screws to the acrylic plate.