I know the original Gibson 60's version of the ES-12 used the same width neck as a standard ES-335 1" 9/16th's or 39.68mm which a lot of people found too narrow for a 12 string.
Gibson's relatively recent small run of reissues of this ES 335 based 12 string fixed that problem by widening the neck at the nut to 1" 3/4 or 44.45mm
I just measured the fretboard width at the nut of my ES -1 G kit and it's 42.3mm
Which measurement is the ES-12G kit ?