What Belongs Here?

I'm trying to put the A/C unit back into my 1973 Dart Swinger since I live in Florida and it was disconnected when I bought it.

My question is what covers the opening in the firewall where the evaporator coil tubes come through? :dontknow: (See 1st picture)

My service manual (see 2nd picture) shows a keyed triangular plate adjacent to the expansion valve that seems to attach to the firewall using 2 screws. But if you look at my 3rd and 4th pictures, you'll see that the triangular plate isn't enough, by itself, to cover the firewall hole and my firewall does not have screw holes in it to accomodate the 2 screws shown in the service manual picture, even though the triangular plate was with the parts the previous owner took off. What gives? ](*,)

Does anyone know the correct way to seal this hole up? A photo or 2 of one properly done would really be helpful. Your assistance is greatly appreciated. :help: