On page 14 of the AR3 manual the J1 platform does not have a recess on the right side. Mine has a .25 recess where the 3 M6 bolts screw in.

My J1 platform has also wrong shape, I think. "Houston we have a problem"

The only thing I can think of is a design change because the new motors and the encoders might be different?

Yes must be better desingn this. In cad file its just like in my previous picture...Houston can sleep

Sorry I just saw this post - yes that was a design update to correct a minor offset that I wanted to improve. no issues.