Block and Tackle rod Inquiry

through watching your video about converting a Crossbow Balance to a Channel balance, I believe my current balance is a Crossbow balance. I believe I need a Swisco Series 795 T Lock Channel Balance. My difficulty is in selecting a Stamp option and balance length.

Stamp Option
The window weighs 19.4 lbs, which would normally require Stamp Option B (" 16 to 22 lbs"). However, with regard to Stamp options your website indicates that the "information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight." Under the current set-up, the window jamb or frame provides no friction. If I lift up the window sash and then withdraw the support, the window sash will drop without any resistance. Does this mean I need a different Stamp Option? If so, which?

Balance Length
My second issue comes in selecting a balance length. My current window balances have an unusual set up (see photos attached). They have no channel surrounding the balance. The balance consists of a T shoe at the bottom. This shoe is hooked to a large spring. Inside the spring is a long and hollow rod which either sits completely inside the spring, or protrudes (in part) from the top of the spring depending on the position of the window sash. A piece connects the rod to a string which travels up from the top of the rod and is secured by a screw into the screw plug of the window frame. Because this set up is so different from the channel balance displayed on your "Series 795 Channel Balances" page, I am not sure how to figure out what the balance length I will need is.

Any help with these issues will be greatly appreciated.