Jim,
"Err, that would be nominally 2 inches by 4 inches, which is really 1.5 inches by 3.5 inches.... Ok, so maybe there IS something to this metric stuff."
While I'm comfortable with both also, I don't think the metric is better in the quote above. I smile as I say that, not nitpicking.
Our 2 X 4 was AT least a full 2 X 4 many years ago, and has steadily shrunk as mills decided they could make more money from a given quantity of wood by selling smaller pieces for the same purpose at the same or ever escalating price.
Ask some of the older European members if they remember what their 50 X 100 mm studs were way back then, and if they are now sold as the same but measure 37.5 X 75 mm. Maybe that's a 5 X 10 cm stud, instead.
I'd eyeball the keys into approximate sizes then use a set or capscrew to finalize the sorting. If you have bigger ones without a capscrew to fit, 1/4 or 3/8 drive 6 point sockets will work They make them in metric, too.
I gotta admit, my own are piled into a toolbox with nothing else in it, not more than about 10 pounds. When I need one, I eyeball and try on a sample cap or setscrew. You get down to an .050, you probably should mike it.
Cheers,
George