America's Tire didn't have the answer...
 

Nor did a couple other name-recognized tire stores.

On my 16 inch stock Porsche Boxster wheels... on either side of the valve stem, and raised-relief upon the tire not the valve stem itself, is a number. Thus, two numbers separated by the valve stem.

On one set of the wheels the numbers are 6/50.

The other set, 7/40.

What do those numbers represent/mean?

Thank you.

6 inches wide, offset 50
7", offset ET40

Thank you!