Window balance part needed

Question

Hello, I am replacing several balance pieces.  They are 27" long.  The only identifying marking is the date of manufacture and "unique balance 26d"

Bob · Accepted Answer

Hello there. This looks like it might be either an S378-27, S380-27, or S385-27 Channel Balance. The only difference between these three balances is the 'depth' of the bottom nylon shoe. See below. Measure your old balance's shoe depth to see what matches best. In any case, for a stamp option choose 2640.

Comparison of the S378, S380 and S385 balance shoes.