Those were part of the slow introduction of "foreign" cars to the U.S. market - mechanics needed metric wrenches, but many of them were reluctant to do much work on that alien stuff, and sure weren't about to get a full set of metric wrenches for cars that might not be around, since American Iron Will Rule.
Then the American makers started mixing metric and fractional-inch fasteners on their cars...
Haven't seen one of those double-marked wrenches in years. They never did work well, and they were always (at least in the U.S.) fractional wrenches with the nearest metric "fake", rather than the reverse. Me, I grew up in a foreign car family, and it never seemed all that weird to have sets of both. There's a great scene in a comic strip called "Travels with Farley," and later just "Farley," drawn/written by a guy who lived in the next county south, in which Farley discovers that his pet raven Bruce has a full set of both metric and SAE wrenches and sockets and is suitably awed. I bet that, if you showed this particular comic to a young mechanic, s/he would wonder what the heck the joke was.
