More pictures showing how my Corbin seat is attached, if it helps.
Front most pivot pin on right side of the frame. The pivot pin is "S" shaped and you can just catch the 90° bend in the pin as it disappears behind the tail piece. That pin is welded to the frame. It should have a circlip on the end but I just can't get the seat hardware adjusted properly to attach the clip. It works fine without it.:
You asked what you're missing. The pin shown in your picture labelled "Seat 3" is the remnant of the pivot pin that should be welded to the frame. It appears to have been cut off and left in the hinge piece, from the looks of things. I think you will have to manufacture a brand new pivot pin and weld it to your frame.
The latch and the front most pivot attachment with the seat stay:
The rear most pivot hardware. On mine, there is a pin that goes through and gets secured with a circlip. To remove the seat, I first disengage the seat stay, then I remove that circlip, push out the pin towards the front of the bike and lift the seat off the front most pivot.:
Another closeup of the front most pivot hardware showing the seat stay secured with a circlip:
The seat stay collapsed with the seat in the closed position: