Andersen Double Hung Balance circa 1977

A quick learner says:
Hi There,

I have an Andersen E40 / 9#31 8/77. I'm guessing 24-939 is the correct replacement? Will the replacement fit the existing space without modification? (see photo)

In general, with the older boxes, is it safe to assume the number before the # is the weight, and the number after is the length? For a replacement a matching weight and a length equal or greater should do the job?

Thank you,
Thom
User submitted photo of their E40 Andersen balance box.
Profile picture of Mike
Mike from SWISCO responded:
Thom,

The 24-939 Andersen Sash Balance is the direct Andersen replacement. The installation screws might be in a different spot, but other than that there is no modification needed.
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