Channel length is 26 5/8". Looks like and S385-26 Series.
26-4 (See stamp photo)
Windows were new in 2002. Summit 7060 Series Vinyl Windows (Jeld-Wen family).
I have (10) 3/0 x 5/0 SH windows for starters. The sash weight is approx. 23.5 lbs. so the 26-4 is appropriate according to your scale. However, these windows were never easy to lift or close (even when new) so I might want to try a pair of 26-5 balances to compare.
Therefore, for now, one pair of 26-4 and one pair of 26-5. Then, I'll order another 8 pairs based on the test result. The odd one can be installed in a window we don't open very often.
Later on, I have (9) 3/0 x 6/0 SH windows and (8) other SH odd-sized windows so I'll need another 17 pairs for a total of 54 pcs.