Top Undated meteric sockets, likely to be near the start. These are hand punched. 

Bottom – 1950 and 1951 SW series (Very early metric)

Photo courtesy of Ken and Roseanne Johnson

The Metric System

The French came up with this system around 1800. My personal view is they did this because they are French, but others feel it was an attempt to unify all the local measurement systems in Europe. A meter is actually defined as 1/10,000,000 the distance from Equator to North Pole. Of course they were French so it isn’t, but they thought it was. I think there may have been imports from the Benelux region in the 1930s that required metric tools and this first start to appear in very small numbers at that time.  They really don’t find US popularity until the 1960s with Japanese imports.