Screen window retaining clip
A handy person from Corlette NSW Australia says:
January 2, 2015
This small plastic part is used to retain screens in the frame. The screen frame rides over the raised section and is held in place.
Dave from SWISCO responded:
January 5, 2015
We do not carry a clip like the one you show in your photo. We do carry clips similar to yours, but you will need to fasten them with a pop rivet. Ours do not have that attached pin to the clip like yours does. Take a look at our 70-014 and 70-026, along with fastener 33-005, and see what you think.