Broken window track

The broken window part you showing us is known as a channel balance. Two come to mind when looking at yours: our S351-21 and S390-21. These look almost look identical with the exception of the bottom balance shoe. The Series 351 comes with a 1" shoe and the Series 390 comes with a 1-1/8" shoe. You will need to measure the depth of your bottom balance shoe like in the diagram below to determine which balance would make the better match. Whichever one chosen will go with a 2040 option. 

Comparison of 15-001 & 15-002

Can you confirm that measurement? A 1-1/4" depth would make it a truly massive shoe. I don't know of anything made in that size.