21 4 lp Caldwell Window part

Thanks for posting. Can you confirm the length of the metal channel? It should be 22", not 23".

As for the Series, that depends on the style and size of the nylon fittings at either end of the balance. There won't be any stamp or code that tells you what they are, you have to go by their physical features. Look at our S385-22 or S380-22 to start and carefully compare the nylon fittings that come with these balances with the nylon fittings on yours. I feel confident it will be one of these two, but you'll have to look them over to confirm which.