Only one plug that goes in the visible threads. The hexagonal hole is down there, you can just make out one side if you look close. The O-ring I'm referring to isn't visible unless you unscrew the top tube.
Leonard:
You have really clarified some of the questions in my mind. Now I see what you mean. I did see a part of the hexagonal hole, deep down in your picture.
In one of my pictures in this post (SDC13951) that shows the exploded view of fork, the O-ring you are referring to is No. 2 in the picture ? I could not find where was this O-ring on my bike until you described with picture. Thanks.
I do have one question though - If the Top Screw Plug is long enough and its tip reaches the hexagonal hole and covers it fully, how can oil come out and reach the O-ring (no.2 in picture) and then slip outside ? The reason I am asking is that in the other picture - with big hex hole (SDC13947A) - the edges of hex hole show a circular marking as if the tip of Top Screw Plug was touching it and blocking it. When it became a little loose, the contact was broken and oil came out.