Pletscher and Esge sold these kickstands in two versions. With a top plate, and supplied with a hex bolt like yours, and with a braze-on plate with a socket cap screw threaded to the head.

Either way you need a 10mm cap screw threaded to the head. I believe it's a standard M10x1.5, but can't say so with any assurance. I suggest that you bring your bolt to a hardware store and compare the thread pitch. But what is key is that the screw must be threaded up to the head (or very close) since the plate is thin. If you cannot find such a bolt you'll need to add some washers under the head to take up the length of the unthreaded shank.