One online source gives the year as 1940.
Contact the manufacturer.
The 3/8 drill is one eighth larger.
http://www.searspartsdirect.com/partsdirect/part-model/Craftsman-Parts/Grass-trimmer-weed-wacker-Parts/Model-536773510/0247/1503820?searchedModel=536.7735.100&blt=
http://www.managemyhome.com/mmh/lis_pdf/OWNM/98110089.pdf
by using a drill
They drill water wells, usually with a truck mounted 6 inch drill.
8mm is almost exactly 5/16 inch. The wrenches are interchangable.
The recommended drill bit size for a 15/64 inch hole is a 15/64 inch drill bit.
The recommended drill bit size for a 5/16 inch screw is 1/8 inch.
A 4 inch drill pipe torque is used for fishing in small casing.
For coarse threads (13 threads per inch) use a 27/64 drill
The recommended size of the drill bit for a 7/16 inch hole is a 7/16 inch drill bit.