Yes -can confirm part no 90179-18006 is the 12mm size-just measured to check!
Just discovered that I have both:
The Kit 90891-10124 with the 12mm Nut and washer in it and the Kit 90891-10124 with two separate packages in it.
The part numbers packaged in two separate packages within the package are:
90215-21290 Washer
90179-18006 12mm nut
See pics, both are 12mm and the only difference in the two is that the individually packaged nut in the package is shiny