Plastic clip that holds the spring loaded rod down with the sash.
A home owner from South Carolina says:
September 15, 2014
I don't have a picture of the part in its entirety, every part in my house is broken. It is clear, and holds the spring loaded rod in place with the lower window. The windows were installed in the house in 1988 and have no markings, name or numbers. Thank you for your time.
Paul from SWISCO responded:
September 16, 2014
Hey, Korey. This part is known as a top sash guide. Yours looks similar to our 18-118, but check it out and carefully compare it to your own hardware to be sure.