Ok, this is what I get for taking my truck completely apart over 2.5 years ago. I can't for the life of me figure out where this piece belongs. I can't find it on the vendor's sites, and the 72 doesn't have one, it seems. It was in with my steering column stuff.
The rubber seal with metal ring screws to the inside of the firewall, and the steel support piece bolts to the outside. This seems to belong somewhere in between, but I can't figure it out. Pics or an assembly diagram would be super helpfull.
Derp.