Aluminum windows approx 26 years old

Hello there. It appears the fittings on both ends of the metal channel lack 'wings', or side tabs. Is that correct? If so, then this is either an S351-31 or S390-31 Channel Balance. The only difference between these two balances is the 'depth' of the bottom nylon shoe, or the heel-to-toe measurement. See the diagram below for reference. Compare both balances with your old hardware to see what matches best. In either case, for a stamp option choose 3040.

Comparison of the S351 and S390 balance shoes.