Tom,
The length of the metal on the 1940 is definately 20". Unfortunately the channel guide for the other side has gone missing. What else would you recommend or do you think is is better suited for a repair pro to look at?
The guide might not be missing. These balance systems are asymmetrical, you see, so both sides are different. For example, the other side may have a shoe at the end of the balance and may even have a different stamp. You should double check and let me know what both stamps read, as well as give me an idea as to what attachments you have on both balances. If you take a picture of both ends of each channel balance so we can see for sure what's missing and what's supposed to be missing.
Tom,
The only channel balance I have from the side with plastic hinge is the 1940 as pictured. The only attachment left is the top attachment. This is like putting together a puzzle with missing pieces. If I had those other pieces this will be easier -)
BTW would the missing balance have a lighter or heavier weight rating?
Is there perhaps another window in your house that would have identical balances? I'm afraid that I do not know what stamp your other balance would have. That seems to be the last piece of information we need, though.