Linear Rails what are these numbers?

I’m pulling apart some old and heavily used linear rails to rework them for a project. I’ve noticed that on the underside of each carriage is some sort of negative offset ie. -2 , -3, -5… My guess is that this is some sort of tolerance note on the block dimensions, but for the life of me I can’t find out what exactly it is… At this point curiosity is literally driving me nuts, and two hours of google have yielded nothing, probably because I have no idea what I’m searching for. So who knows what it is and who wants to guess?