A quick learner from Gainesville. FL says:
April 19, 2016
Stamp # 1820. Measures 18.5". Non tilt window. Half inch channel width. Appears to be a 377 series but spring in mine connects on top to a bar inserted 14.5" from bottom. It doesn't extend full length as shown in your photo. Alum window installed approx 1974. Just want to make sure I order the correct balance or before ordering. Thank you for your help. Jim
Paul from SWISCO responded:
April 21, 2016
I agree, our Series 377 looks like a great match. I wouldn't worry about the length of the spring. Different manufacturers do this differently. What matters is that the strength is the same, which is indicated by the stamp. Based on yours, I'd go with the 1820 option.