Appears that the standard RotoPax
mounting handle is simply attached to the black plate on the frames.
(See photo...2 bolts in center of black plate...handle is on the inside facing the tire).
If that PLATE isn't standard to the frame...was that your question???...then it looks like the owner simply added those 2 brackets at the top and bottom of the frame and then attached the black plate. Otherwise...don't know.
Hope this helps.
Here's a picture of the RotoPax mounting handle...it's facing inboard towards the center of the bike in the installation above. Comprendo?
The black circular plate on the handle is attached to the black plate with the 2 bolts in the center of the black plate.
You can't see this handle in the first picture.