Weather strip between panes of a dbl hung
I'm afraid I can't identify what you have from this photo alone. In order to help, I'll need you to remove a sample and show me a clear picture of the end profile. The video below will clarify.
I know you said that your seal has disintegrated, but we need to see an undamaged sample. There is no other way to ID it.
This does help, yes, but the bad news is that we don't have a type of weatherstripping that matches this design. The closest options we have are maybe the 58-171 and 58-182, but I cannot guarantee that either will work.
My recommendation is to look over the dimensions and photos we show for each of these products to determine if you could use them to substitute your original seal.