There might have been a hose that ran from that fitting around the back of the cam cover and forward again to a 'TEE' connnected to a heater hose. It would allow coolant to flow thru the bottom of the intake manifold to warm up the incoming mixture for better combustion. The hose is about 3/8" in diameter.
If you look inside, does the intake have that coolant-dye color?
tom