Tilt-Window Spiral Balance Question

A quick learner from TN says:
April 14, 2020
Have removed old balance and is 29" long (tube only) But the diameter is 1/2", not 3/8" or 5/8" (please see photo). Can I use a different size, and if so, which is best?
Thumbnail of the uploaded file named Balance half inch tube diameter.jpg
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Bob from SWISCO responded:
April 16, 2020

I believe this reads as 9/16". It's hard to tell with these spiral balances that have older plastic tubing, since they warp over time; but I think this is likely an S600-29 with a Black Tip option; but double-check to be safe. 
29" Series 600 Spiral Tilt Window Balance
29" Series 600 Spiral Tilt Window Balance
S600-29
$9.35
29" Series 900 Spiral Balance
29" Series 900 Spiral Balance
S900-29
$9.15
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