I have a Greenlee chassis punch for the double D outline of the holes in a US standard duplex outlet (115V, 15/20 Amps). You are probably right about the original dimensions of these outlets being in fractions of an inch, but that was most likely for the outlets themselves and not for the cutouts in the covers, which had to be just a bit bigger. And perhaps things changed a bit over the years.
This is my working drawing which I use whenever I use that punch to create a mount for the duplex outlets, which I have done a number of times. The dimensions on it are from a combination of my best guess at the original, fractional dimensions, measurements of the chassis punch and the holes it makes, and from measurements taken from a number of different brands and quality levels of outlets. They DO WORK. If you round to fractional values, particularly if you round down, better have a file handy to make the outlet fit.
Anyway, for what it is worth, here is my working drawing. All dimensions are in decimal inches. Some are for fractional values and others are not.
And perhaps, just perhaps, with most of the outlets being made overseas, the manufacturers are rounding those original fractions to more even numbers in mm. So 3 1/4" becomes 82.5mm which then is 3.248". And there goes your fraction.
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