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? Well, I listed them all because there isn't much info on the internet when I searched up each one of these.
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