Here are the pie plates; aka pop in deck port plates:
http://www.beckson.com/prydp.html
As you can see on the clear one, the center plate has a black O-ring that is around a lip on it. The center plate snaps into the base. You can see the pry point (flat area) that a pry tool or flat screw driver is installed to pop out the center plate.
Here are the screw in deck plates. The center plate has about 1.5" of threads and you screw the center plate into the base. It also has the black O-ring to seal. You can tighten them down fairly well.
http://www.beckson.com/screwout.html
Beckson sells replacement O-rings as well.
For some of the newbees, some advice.
The screw-ins are direct replacements. You unscrew the bases and remove. Cut out old caulk and clean well. Then either use the silicone or 4200. Personally this is a no brainer in my opinion - especially if you are off shore in any weather. The euro transoms are prone to wash over, especially on the drift in rough weather. Why Grady and people with boats wouldn't change is a mystery to me. Pretty inexpensive and easy to do.
I recommend getting yourself a bag of wooden thru hole plugs, a few containers of "stay afloat" and put on the top of your ditch bag.
http://www.youtube.com/watch?v=888wQhRcmjQ
It is also wise to keep all seacocks closed when not in use and check periodically for proper operation. Sometimes these can freeze or become difficult to move. A little preventative maintenance goes a long way.