It's a center mounted drawer slide, and the part of the rail that screws on to the bottom of the drawer has a plastic part at the back end. It's a kind of T shaped piece that fits down into the bottom rail and helps lock the drawer into place, so it doesn't rattle or fall out of the rail completely.
Tom from SWISCO responded:
May 1, 2017
Thank you for your post. Please take a look at our 32-078 drawer guide and let us know if this is a good match for you.