Norco window channel balance--S751-20?

A home owner from Merrimack, NH says:
Greetings form New Hampshire, I need to replace the channel balance for two Norco windows manufactured in 1990"s. Moreover, the interior ( lower sash) channel balance are stamped 20 HDB but the total length is 22"! While exterior is 21"stamped 20 HD. Therefore, which part number shall I use? Is there 20 HDB in stock that indeed has a total length of 22" for interior sash.
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Tom from SWISCO responded:
That is odd. When you're measuring the length, are you making sure to only measure the metal channel? A stamp that starts with "20" should be on a metal channel that is 21" long, not 22".
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