4.6 liter freeze plug question

Hello everyone, I have a slight problem with my girlfriends 1992 ford crown victoria, the rubber block off for the heater core broke and she overheated.
I replaced the plug and the car leaks coolant from between the engine and transmission, I checked to make certain the leak was not coming from the outside of either head or intake and everything there was dry.
I removed the transmission and flywheel from the car and I saw 2 clean freeze plugs on both sides of the block in the rear and one open hole in the center top, also I could see a big stain of rust from there so I assume the freeze plug must have been forced out of there by the overheating pressure, my problems are I've never taken apart a ford and I never seen a trace of this missing freeze plug in the bellhousing area or the transmission .
Is this top empty hole in the back of the 4.6L ford block for a freeze plug or is this something else?
Any help would be greatly appreciated as I need to finish this job for her.
Thanks
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