A quick learner from new smyrna beach florida says:
September 19, 2019
window balances with marking "28 40" 1/2 inch wide and 29 inches long ( metal part )
Bob from SWISCO responded:
September 24, 2019
This looks like either our S380-29 or S385-29. The only difference between these two balances is the depth of the bottom nylon shoe, so compare both with your old hardware to see what matches best. You will need a stamp option of 2840.