Searching for Bsi 21hd

Good afternoon - I have several broken BSI window balances at my house, where there is no tension to keep the windows up. I am trying to replace these parts and need to identify how to determine what part and how to remove the old ones. I have no idea what kind of vinyl windows I have, they are approx 20 years old and no identification on them. The inner balance has 21HD and the outer balance has 19HD. Any information would be great! Thanks in advance!