What is this weatherstrip piece

[mplex2000]

After doing my relays I realized just how bad the weatherstrip was on my 79. I went ahead and ordered up a kit, thinking I would skip the vent windows because they scare me. Then I thought better of it and picked up the vent window kit locally

After looking at numerous articles and videos on how to replace this stuff I busted everything out and I have two pieces they I'm unsure what to do with. The attached pic shows, from top to bottom:

1. The channel piece I removed from one door
2. The new channel piece
3. Mystery straight piece with metal tab

The vertical strip on the vent window (at that main window rides in) came out in tiny chunks so I have no pictures of it, but it appeared to be a separate piece from the rest of the channel seal.

Now I'm confused, it this straight piece to replace that one, or ? It looks like the main channel piece should run all the way around. Maybe this is a "snip off and use the straight piece if your truck had a two piece before" type deal...wouldn't know from the instructions that basically say "push the channel piece into the slot, the end".

Hoping you guys can guide me in the right direction here!

[Pontiac1976]

If I recall right it the seal to the lower channel of the vent window. Do not cut it. Start @ the vent channel work your way up a cross the top & down to the door handle.