WSW-105 USA 4 Snap on 9/16 W
WSW-104 USA 3 Snap on 1/2 W
WSW-103 USA 2 Snap on 7/16W
WSW-102 USA 9 Snap on 7/16BS
WSW-102 USA 7 Snap on 3/8W
WSW-101 USA 0 Snap on 3/8BS
WSW-101 USA 2 Snap on 5/16W
WSW-100 USA 3 Snap on 1/4W
WSW-100 USA 6 Snap on 1/4W
So, what do these have to do with what? My friend's dad was a British car mechanic, and when he died, she gave me his tools.
They are used on old Jags, these tools with the strange and incomprehensible markings were once common enough that even Sears Roebuck and Snap-On carried them.
Before the mid-nineteenth century, nuts and bolts were individually hand-made, specifically matched and were generally not interchangeable. Serious efforts to standardize screw threads began in 1841, when Sir Joseph Whitworth proposed a standard screw thread form based on a constant thread angle of 55 degrees. This became known as the Whitworth thread, and gained acceptance in British industry. Outside diameters of bolts began at 1/8" and increased by fractional inch increments, with a whole number of threads per inch specified for each diameter.
In 1965 the British Standards Institution approved a policy statement urging British industry to regard BSW, BSF, and BA as obsolescent, to be gradually replaced by International Standards Organization (ISO) metric thread
I learned a lot from http://www.jag-lovers.org/xk-lovers/library/whitworth_system.html