Plastic guide on top corner of window pane.
A handy person from Florida says:
November 14, 2013
Double hung window has no mfg. markings or stamps. The balance rods are marked Caldwell. Length is one and a half inches and one half inch wide
Paul from SWISCO responded:
November 18, 2013
This looks a lot like our 18-166. Carefully compare the dimensions we show to your own hardware and let me know if you have any questions.