Plastic shower door guides.. Screw into the bottom of the door, 2 7/8 x 1 1/2 ".
A quick learner from Matthews nc says:
March 30, 2016
No markings, screws go into a vertical opening in the plastic guide
Paul from SWISCO responded:
March 31, 2016
Without more information, I'm not sure what to recommend. Could it be our 10-139? If not, can you show us some pictures of your part?