What is this

It is the OSAC valve. Yep, it's part of the emissions control system.

The OSAC valve has been used since the early 1970s to delay the distibutor's vacuum advance for about 15 seconds during acceleration. The OSAC valve is on the air cleaner (pre-1974 models had it on the firewall!) . One hose runs from the vacuum of the distributor to the valve, the other from the valve to the carb. Running the hose from straight past the valve will eliminate its limitations, and compensate for a bad valve.