Not neccesarily true... For instance, the steering damper on my M750 wouldn't work on any S4 because the radiator is in the way. Some above the clamp dampers also may have issues with cast-in bar clamps.i don't think you'd have any fitment issues using a damper marketed toward other monsters.
frame tubing's the same diameter as are the fork legs.
Yep. It's an Extreme Tech damper and it goes from the lower clamp on the right fork leg to the voltage regulator mount on the left side of the frame. It's basically a transverse mount damper that mounts below the lower clamp and runs across the frame right where the radiator is on a S4.fillmore, is yours an under-head damper? if so, it would conflict with the radiator? never took that close a look... thanks for pointing it out if this is the case.