http://www.wrenchingnews.com/wrench-logos/logos-page-1.html
At the bottom of the page is a little article on standards of measurements.
"SAE Initials: SAE (used with size in inches of bolt diameter i.e. I/2 SAE)
Society of Automotive Engineers (fine threaded nuts and bolts)
Found on older forged wrenches before the 1929 standard want into effect"
"USS Initials: USS (used with size in inches of bolt diameter i.e. I/2 USS)
United States Standard.
Found on older forged wrenches before the 1929 standard want into effect"
These are the two I most commonly come across and occasionally you see, "OC"
"OC Hex logo and initial C (used with size in inches of bolt diameter i.e. I/2OC)
Hex Cap Screw
Found on older forged wrenches before 1920 standard want into effect"