Late to the party, but for future reference...
This device was used on the '90-'93 1.6l engines in the US, and probably elsewhere. It is a thermostatically-controller air valve, which sits inline with an air passage cast into the intake manifold, which draws are in from a bypass hole cast into the throttle body flange, and dumps it into the plenum.
It is controlled by the temperature of the coolant flowing through it, allowing a bit of extra air into the intake when the engine is very cold. The factory manual test procedure calls for it to be cooled to 32°F, but does not give an actual spec for what temperature it normally opens and closes at in typical operation.
A lot of folks think that this valve was unique to the 1.6 Miatas, but the '94-'97 cars had it as well. On that application, it was integrated into the lower section of the throttle body, in the same housing as the IAC solenoid.
It's the part labeled "Thermowax" and "Air Valve."
Originally Posted by skeeler
Can someone please give me some information on how to do that? I know Trackspeed offers a freeze plug for where the neck is, but what do I do with the other plumbing?
Just remove the hoses from the throttle body and air bypass valve, cap them off with rubber plugs (optional) and install a new hose between the port on the lower intake manifold and the port on the thermostat housing (if present) or mixing manifold where the hoses you removed from the throttle body and air valve came from.