peachtree non-tilt double-hung sash balances

I have many Peachtree double-hung windows at my home, installed in 1985. The cords that operate the balances snapped in a couple of the windows, which now need to be propped open. Several pictures of a balance installed in a sash are attached.
The balance U-channel is 20-1/4" long and 5/8" square, and attached into the sash with a small screw at top and bottom. There is a plastic plug the inserts into the jamb to operate the balance.
The sash glass dimension is 30-3/4"W x 20-1/4"H and the sash weighs 15.3 lb without the wood trim.
The sash balance is stamped with the number "3648" and also has the small letters "B.S.I." stamped on it.

If you have balances that match these descriptions, I am interested in purchasing a number of them, two for current repairs, and some for future back-ups. I believe my only alternative would be to try to re-tie the string in the balance, which looks like it would be a pain.
Thanks.

Are the S745-20 and the 19.25-4 two different balances, or versions of the same balance. I would go with your recommendation, as long as the dimensions match the ones I sent. Would it be helpful if I sent some more pictures? If so, what would you like to see more clearly? Thanks,

Dave