dusty wroteJPG wroteTo be successful, the first goal is to get the two end castings aligned to each other(in the two horizontal planes) before even considering the aux table.
Since Ed was successful when after he converted to the 520, that must have been done.
If that be the case, what is left that could prevent extension t6able alignment (interchangeability)?
The Base and Base Arm Assembly maybe????? Hmmm - Maybe a double tilt with two Headrests.
Ignoring possible unlevel floor issues, The headrest is the culprit with the single tilt models. That huge set screw that has been touted(BY SS) as a method to align to both sides ignores the intended purpose of that screw and also ignores the actual cause of 'the problem'.
That setscrew should ONLY be adjusted to insure the way tubes are parallel(not twisted) to the bench tubes(floor). That twist creates headrest/table rotation as they are moved left to right/vice versa.
Then the castings need to be adjusted to be parallel(spt holes to opposite end spts hole) and the castings holes equidistant between the ends.
The dual tilt castings almost guarantee that.