Bottom of closet Door plastic "insert"
A home owner from San Carlos, CA says:
October 6, 2014
This "insert" fits within the sliding closet door bottom and contains a spring loaded peg that protrudes along the bottom edge of the door and uses the spring pressure toon the peg that pushes downward to keep the bottom of closet door into a metal channel. The insert is encased inside the door ends, and a single screw hole(actually allows adjustment of the insert up or down) in the door. These are old sliding doors of a closet, in a house built around 1975.
Tom from SWISCO responded:
October 9, 2014
It's hard to tell since it appears to be a little damaged, but have a look at our 23-235 and let me know if this could be a match.