Why are channel balance options printed like this?

Thanks for posting, Shujin. This is a good question.

For this type of balance, the stamp options we have available do not indicate the length of the channel. In fact, the first two digits of the stamp (in this case, 32) are one digit less than the channel length. So a balance whose stamp is 29xx would have a length of 30", or a balance whose stamp is 15xx has a length of 16".

This rule is not universal for all channel balances, but it is pretty typical for the variety used with non-tilt windows. It's always good to double check, though! Some older balances may have been cut down or altered in some way, rendering this rule of thumb relatively useless in those instances.