channel balances

I have several windows with the same problem. Either the spring is broken or the plastic end are broken. The windows are OLD with no markings. I have pictures that I am providing. The key difference between what I have and the picture you show is the plastic piece at the top. The picture will show this, Your picture shows an extension at the top and mine is flush with the metal channel. I am not sure of your measuring system. The metal channel I have is exactly 29" with maybe an 3/8" of plastic sticking out of the bottom, for a total of 29 3/8"
I hope I have provided enough info for an evaluation of the part I need.