Anderson window balancer (windows are from 1977)
A handy person from Mantua, NJ 08051 says:
October 14, 2013
removed balancer and obtained stamped part number on unit: E40 9P27 177
Tom from SWISCO responded:
October 15, 2013
In that case, I recommend our 24-939. This can replace Andersen balance boxes with the stamp 9P27.