Non-Tilt Block & Tackle Window Balance Identification

Can you help me in determining the correct series number for my block and tackle window balances? The metal "tracks" are 28" long, and have a 5/8" outside track width. One end has a plastic shoe with wings on it, and the other end has a shoe with no wings on it. (See attached pictures.) The window balance is stamped " Caldwell 27-4 FQ". Thanks for your assistance with this matter!