[Oldsguy]

That's your heater control valve, most of them are operated by a vacumm signal from your heater/ac controls. It allows water to circulate from your heater core back into the back of the block water jacket. This allows the heater core to really heat up since the water is circulating, if it doesn't circulate the heater core doesn't give off as much heat after a while with the air blowing over it. This is done to keep the heater core form radiating too much heat in the summer. I took mine off when I rebuilt my 455 and I don't notice that much heat (although it ain't August yet). I think they look to big and akward and just plain nasty on a nicely cleaned up engine so I left it off mine. I think in your case you just got unlucky on the second heater core, you have to be really careful when you install one that you don't break a neck when attaching the hoses, done that before myself.