Hey there Tinsmith, idk for sure, but i'm guessing that it's pressed in. The overflow tube above it appears to be threaded, but i'm just guessing there too.
Maybe take a small pair of needle nose pliars, go straight into the fitting and carefully apply pressure and gently try turning it to the left, then gently to the right and see what happens. Put a drop of Tri-Flow on it first. If it's not threaded, you'll need to tap some theads for your new fitting anyway, so just gently work it out with the needle nose pliars.
If no one has your answer, that's what I would do, but chances are some one will so hang tight.....