They look like the maximum major diameters for common screw sizes used in models. Here's a little more complete table for SAE:

http://www.engineershandbook.com/Tab...readlimits.htm

The other day, I discovered an important cap screw with a 1/2-12 pitch (Whitworth). It did not come from some ancient British machine. It held the motor frame on a Harbor Freight horizontal band saw, which was bought no more than 3 or 4 years ago. There are two of them. Whatever you do, don't loose one. I suspect the manufacturer in China must have been found a supply left over from the colonial days. Its max. diameter is about the same as a 1/2-13, just a deeper cut thread, in case you want to add it to the table.