What is this??

It's part of the emissions deal that was on every 1971 Mopar car in some shape or form and all 1970 California Mopars.

The Factory Service Manual has more description of it.

There's about 4 hoses on the bottom of that. One of the ports is connected to the vent line that runs the body, goes up the passenger fender and connects with the 3 nipple valve cover breather.