Found Block Numbers

Hers are the numbers.

I waqs using a honing stone on the deck of my block when I noticed numbers at the bottom lip of the cylinders where the deck is the thinnest. Some are zeros and some are -5... What do these numbers mean? I have measured all the cylinders and they are +5 thousands from the standard piston size. I have a new set of pistons in this engine too from 1961 when it was supposedly rebuilt. Just wondering what I have found. If a pic would help I sure can post one.