OK, let's see if I can cut through the confusion here.
The bolt that goes through the rubber block has no nut - and in most cases shouldn't even be threaded except for the part up near the flange (as in photo of Paul's bike). It actually isn't even shown on the parts pages for the stand, as it is classed with the exhaust. If it had been replaced at some point, it's possible that a fully-threaded screw was used because it was handy. In any case, it has no bearing on the stand plate.
The two bolts that fix the stand's plate to the block are 12mm x 35mm dome-head socket drive machine screws. They do not have nuts assigned, since they screw directly into the block. If your plate is loose, you'll need to use an 8mm hex key to tighten them. If they are quite loose, back them out and apply new loctite. Unfortunately, the exhaust header pipe blocks access to the upper of these two screws.
The upper screw is visible behind the exhaust. The lower one is recessed inside the tube that projects from the stand plate (part 11 in the diagram, partly obscured). The lower one will require a long key or key socket.
