Identifying channel balance size, series 380

A handy person from Williamstown, NJ 08094 says:
Trying to identify my size. I have two different window sizes.
1. Balancer identifying marks 27 1/2 3 A I Metal measures 28 1/2"
2. Balancer identifying marks 29 3 J H Metal measures 30"
I have identified the series by the samples on this site.
Can you tell me the correct items to order?
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Mike from SWISCO responded:
Our balances measure on the full inch. I am not certain whether the 28 or 29 inch balance will work for the first balance listed.

Please list the sash weight so we can recommend a stamp option (remove the window sash and place it on a scale to determine its weight).

A quick learner from williamstown, NJ 08094 says:
window sash weight is 20 lbs. I think the 28 inch balancer will fit better. The 29 inch (I think) will be too long. Thank you.
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