what is this?

I've got a '72 factory manual and I can't find a vacuum diagram. They only mention EGR almost in passing but state it comes up through the floor of the intake by an orifice. No mention on an EGR valve but yours sure appears to have one. In 73 and 74 the vacuum for it usually came from the carb, through a temp sensor on the top tank of the radiator and through a vacuum booster before reaching the EGR.

As stated in the '72 factory manual:

"The exhaust gasses are introduced into the intake manifold through jets in the floor below the carburetor. An orifice in each jet allows a controlled amount of exhaust gas to be drawn through by engine vacuum to dilute incoming fuel and air. The illustration shows the configuration used in 8 cylinder engines where exhaust gasses are taken from the intake manifold exhaust crossover passage.

In six cylinder engines, gasses are taken from the exhaust manifold "plenum" chamber located at the "hot spot" below the carburetor riser."