27-3, but 28"?

A home owner from Surprise AZ says:
Three of my bedrooms have slide up windows measuring 35" x 58 1/2. The channel balance I measured metal portion end to end at 28". The markings on the metal are: 27 3 HW CMC 11. I've included 3 photos to make sure. I've YouTubed some videos on how to remove window and install the channel balance. The stamp numbers 27 3 seems to conflict with the 28". Help Thank you
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Bob from SWISCO responded:

This is actually typical of non-tilt window channel balances: the first two digits of the stamp are usually one less than the length of the metal channel--those two digits refer to the height of visible glass in the window, which is usually 1" less than the metal channel length. So you have either an S351-28 or S390-28. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. For a stamp option you will choose 2730. 

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.
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