There seem to be two standard spacings for rack mounts, 5 and 8 cm. Many, but not all, racks simply have two slots in the plate to accommodate both. Some lights let you select which spacing you want to use by eg. inserting bolts into one of two pairs of holes, but with others you have to select the correct model at purchase. There are also racks with a plate which has only two holes fitted vertically above each other (I think Blackburn racks are like this), and others with no mounting plate whatsoever. Some standardisation might be beneficial for us poor users of these items.