What's this part called.

No, it's not the OSAC valve—that's not on the air cleaner; in '73 it's on the passenger side of the firewall (and should be bypassed; run the hose directly from the carburetor to the distributor without stopping at the OSAC valve, but that's not what was asked).

What you're talking about is the thermostatic air cleaner system. You can read about it here. If your temperature sensor (flying saucer lookin' thing on the floor of the air cleaner housing, inside the filter element) has gone bad, as they eventually do, send me a PM; I have some new replacements. If your snorkel flap actuator (round pod perched atop the air cleaner snorkel) has gone bad, you'll have to find an air cleaner housing with one that hasn't; replacement actuators stopped being available long-long ago. If your stovepipe (flexible duct that runs from the bottom of the snorkel down to the sheetmetal 'stove' around the central part of the exhaust manifold) has torn or crumbled, those are readily available universal repair parts.