This is not obvious until the valve is lifted up. To look at it installed, and to someone who has never done this before, it looks like all one piece. The picture in the "Needswings" post above I think is correct but it also looks like one piece.
Why can't I post a picture? I did earlier.
I went out yesterday and (I hope) fixed a leak in the valve. When I did the repair I took the two metal canisters mounted on top of the valve off and replaced all the parts inside, (because the valve I received was the wrong part) This worked fine but when I went to take the picture I noticed it had water at the joints. I took it apart, cleaned it. and put a little "Ultimate Gray" on the gasket. It appears to be good now.