I knew I would break one eventually, although it took me almost 11 years. This is the popup stop, that when popped out, prevents the horizontally sliding vinyl windows from opening more than 4 7/8". The visible external measurement of the part is 2 7/16" X 3/8".
Paul from SWISCO responded:
January 16, 2019
Could our 26-354 sash vent stop do the trick as a replacement? Look it over and let us know what you think.