I think Chuck is talking about the rectangular hole directly under the radio area of the lower dash (see attached pic). I don't know what it would be for. Perhaps access to the bottom of an original radio, ventilation, or a brace? Beats me.
As far as the duct mentioned earlier, one end of the oval cloth duct goes to the lower distribution duct attached to the heater box. The other end is stapled on to one end of the small plastic duct. The other end of the plastic duct goes to the small hole cut out of the top back edge of the cubby hole of the console. This is my understanding anyway of 73-80 cars with a console.
If you didn't have a console, then I don't know where the duct goes. Perhaps it attaches to the ultra rare lower dash filler panel that came with non-console cars.
EDIT: My info on the end of the plastic duct going to the back of the console cubby hole is wrong. See posts below.
Thanks for posting that assembly manual page, Dave. That corrects my misunderstanding of where the one end of the plastic duct goes. Not into the back of the 73-80 console cubby hole but on the lower dash where the rectangular hole is located. Well, at least I posted a pic of the rectangular hole in the lower dash.
I saw the page is for a 70 and therefore the 70-72 consoles. I checked my 73 assembly manual for the 73-80 consoles and it looks like the same page (posted below). So the hole or notched out area at the back of the 73-80 is NOT for this duct. I apologize for my mistake. Looks like I'll have to fix mine. D'oh!