Emissions in the 1966 mustang consisted of the use of a PCV valve. Although Ford did use Thermactor devices in 1966, I do not know of any factory installations on Mustangs.
There are two PCV valve configurations and OPEN and a CLOSED configuration.
In an open system ventilating air enters the engine through the oil filler cap located on the rocker cover. The filler cap contains a mesh element which filters incoming air. The air flows through the crankcase and to the P.C.V. valve that is connected to the intake manifold through hoses.
In the closed system, it’s the same as the open except, the air source is the carburetor air cleaner. The air passes through a hose connecting the air cleaner to the oil filler cap. The cap is sealed at the filler opening to prevent the entrance of outside air. A restriction in the air cleaner at the hose connection, assists the P.C.V. valve in maintaining a slight vacuum in the crankcase.