First thing, the vent is clear to the tank. I could actually see the bottom of the tank.
I removed the toilet and separated the halves to look at the nozzle and overflow tube. The nozzle piece (part #12 in the the diagram link provided by KRS) fell out of its hole. The gasket that holds it in place and seals the hole, is for the most part gone. The sewer gases could just waft on up into the pot. Great! I happen to have the right gasket material for making a new one. 15 minutes later and it's fixed. Tight as can be.
While the sealant for the toilet halves is curing, I'm heading for a new floor gasket.
here's the link for the diagram:
Aqua Magic IV Hand Flush Parts