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Its a plenum I believe but not sure if I spelled it correctly. You need one for a 71 ?
edit - on second thought the outside piece is the blower motor case or module and the inside piece is the air distributor I think.
I just replaced mine with used one I got it off ebay for $35 or something. You need to take off the whole wiper door assembly to be able to reach and get the old one out and install the new one in, its not hard, just a little time consuming, it took me about 3 hours to do including the time to re-align everything.
Don't anyone laugh. Sorta did a Bubba on it. Mine was torn part way and I put a strip of black duct tape on each side. Cleaned the surface real well first. It has held up for about a year now. It's on my list for long term replacement, but not in the budget yet.