This is a stop on the frame of an older aluminum window.
A quick learner from Mundelein, IL says:
October 5, 2013
I cannot identify a part number or locate the part anywhere.
Tom from SWISCO responded:
October 7, 2013
I would start by looking at our 18-006. Carefully compare the dimensions we show to your own hardware and see how they match up. If they differ in any way, please let me know exactly how.