Rivco balances

I am looking for replacement balances for my Rivco double hung wood windows. I believe the windows are 29 years old, which is the age of my home. I'm also having trouble removing the balance as I don't see a set screw. The silver part of the balance has the digits "28HD" there is something that looks like a "b" in front of the lettering. Attached are photos. These windows also have storms (although sashes are double pane). I removed the plastic stop (shown in photo 2) thinking there would be a screw behind it but there is not. I also need 10 sets of storm pulls and found those on your site; PN 91-175 and will purchase along with the balances. thanks so much! Carol