YM240D hub extensions:
Flanges are 6.855 across, .867 thick.
Hub is 6.905 wide, outside to outside (including flanges). Accuracy .001
Axle side hub is drilled, other threaded for lug nuts.
Axle side hub recessed to accomodate axle flange 3.716 x .564 deep.
Flange for rim, 3.721 x .289 high.
Inside diam of pipe 2.108
Outside diam of pipe 2.740
In my original parts book, this is part of the 'adjustable rear system' with a reference to it as a 'extension, rear shaft'. This leads me to believe that they were originally meant to increase wheel width rather than serve for dual rim setups.