A handy person from Nashville Tennessee says:
April 21, 2012
Nylon part which holds spring is broken as shown in photo. My window will not stay up. The double hung windows were probably installed around 1986 and I am having a problem finding part. There is no identification on the windows.
Donna from SWISCO responded:
April 23, 2012
Thanks for posting! Please take a look at our 15-104 Caradco friction shoe. Do you think this might match your hardware? If you aren't sure, please try to remove your shoe from the jambliner and take a few pictures of it separately for us to look at and identify.