Yes Stuart, that is a square headed fitting and it does thread directly into the head. There is no 2nd fitting. What you said about sealing mechanically is right, looks like the fitting sandwiches a little flange on the temp. "bulb" between it and the head. What I'm concerned about is how old these pieces are- they probably are not completely smooth, so a leak may occur. So, maybe some of that permatex sealant on that flange would work?
