replacement block and tackle spring balances for non-tilt window

Hi, I am trying to replace the block and tackle spring balances for a non-tilt window. They are supposed to fit into the sides of the frame, but they are completely broken and fell out. The stamps say 27DS B.S.I. EJ 33. They are 28" metal to metal. I have included pictures of the plastic end pieces, as well as the stamp. The window weighs between 48 and 49 pounds. (I weighed it twice - once by itself on the scale, and once by standing on the scale holding it and not holding it to get the difference.) From what little research I did, the heavy window may have been what destroyed these? And they are 22 years old. I cannot find anything online that looks like these with these markings and with that much weight capacity. Thanks, Trey