Leak on back of manifold? ???

I recently bought this car with an LA 360 motor and an Offenhauser intake manifold. It has a small leak i thought was from valve cover but looks to be from a 3/8" brass fitting into a 1/8" line that is on the very rear of the manifold. Tracing the 1/8" line appears to then go into the firewall. ??? Anybody have any idea what that is (or should be?) I was able to tighten it about a full turn and clean it off so I will see now if it is still leaking any.....when I retest I'll get a photo....