Replacement channel balances

A contractor from Seattle WA says:
Existing balances measure 26" long using your system of measure, with 19-005 top and 15-005 shoe. Sash weight is 10.5 lbs. Existing manufacturer stampis "25 1 IT CMC 2". Looks to me like I need a series 380 26" with stamp option of 2520. Two of them. Am I reading this correctly? Why the apparent different length?
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Tom from SWISCO responded:
Thanks for posting. If you have the 19-005 and 15-005 then the Series 380 is a great match. I would also recommend a 2520 option for that 10.5 pound sash. Your original stamp corresponds to the 2510, which is too weak for your sash. Did you ever find your windows difficult to operate before?

I'm not sure about your second question. There appears to be no difference in length. Can you clarify?
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