Channel balance

If the metal channel, not including the nylon fittings at either end, is 23" long, then this looks like either an S380-23 or S385-23. The only difference between these two balances is the depth of the nylon shoe, so compare both with your old hardware to see what matches best. You cite this measurement as 5/8" in your photos; this would be abnormally small, so I'd double-check to be safe.