markings are: 20 4 HN CALDWELL
1/2 (9/16) inch wide. 21 inches long on metal base, not including plastic end pieces.
Pictures attached.
I would appreciate it if you could ID a proper replacement.
Thanks!
Mark

Thanks for contacting us! I'm glad our site is helpful to you. As for your balance, our weight ratings are calculated assuming both balances are carrying that weight. You do not split it in half.
It sounds like it will need to be custom made with a special strength spring. Your sash is heavier than usual. I would recommend a 2065 stamp, which can hold a sash weight of 30 through 34 pounds. Would this work for you?
If so, I've just emailed you a purchase link and price quote for your review. Please let me know if you have any questions!