Hi Peg; hate to disagree, but yes that is a plug. It is threaded into the end of the toilet base, at the end by the toilet bowl. It was put there for (2) reasons: 1) as a drain plug to aid in winterizing the head, and 2) when the actual part is molded, that opening must be there, as that is where the "core" is inserted and removed, to mold the passageway into the part, that runs between the toilet bowl and the toilet pump.
The simplest way to get a removal tool, is to run down to the hardware store and buy a 1/2" hex head bolt, about 2 or 3 inches long (65 cents?). The hex head on the 1/2" bolt will measure 3/4" across the flats - same basic size as the recess in the plug. Insert the bolt head into the recess in the plug. It may be necessary to file it a bit to get it to fit, or to tap it in gently with a hammer, as sometimes the hex "hole" shrinks a bit when it comes out of the mold and cools down. After getting the bolt head into the recess, snap a pair of Vice-Grip pliers onto the shank of the bolt. That's your tool to screw the plug in or out.
