May be a dumb question, but since you're measuring in metric, have you checked to see if they are possibly imperial?

If you haven't already may be worth reaching out to the manufacturer, they may be able to provide you new hardware or at least confirm the specs or point you in the right direction to find replacements.

Failing that, like someone else said, you could try getting a machine shop to make them for you, or if you can track down a local hobbyist machinist that could be another option.

Failing that, you could of course try to modify the knife a bit to work with whatever hardware you can actually get your hands on, enlarging a hole a little or some strategically placed washers can sometimes go a long way.

You might be best served with a machine shop. Knives, especially higher end ones, tend to make their own hardware or use a custom order size that the manufacturer doesn't offer publicly.

Have you looked for the size you need under the name "Chicago screw" or "screw rivet"?

McMaster