That is a dowel plate.

Used for making your own dowels from scrap wood. The numbers stamped should correspond to the nominal drill size, presumably in Millimetres.

You cut short lengths of straight grained wood and shape it to a round size just a slight amount bigger than the hole selected, position the selected size in the plate over a a dog-hole in your bench top then tap it through the plate starting from the flat side with the numbers. It will produce a fluted-sided dowel corresponding to that drill size.

It should work as is, but it seems to have been in a shed for some time? If you intend using it, you may consider rubbing the areas surrounding the holes flat on a very flat smooth diamond honing plate so that the entrance angle is sharp. It will make tapping the dowel easier and give a less rough surface.

If you search ‘Dowel Plate’ in you-tube, it will bring up some videos that explain it.