Paul from SWISCO responded:
January 19, 2018
It looks to me like you need our S380-29 side-load balance. Measure your shoe as shown below. If it matches, you need Series 380. The spring weight option for "28 B 20" is usually "2820." To verify, weigh your window sash, which should come out between 12 and 16 pounds.
Alternate view of 15-005
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