I rebuilt a tuet6400 to replace the tuet6700 that was having vibration issues in my ctsv.
I ended up having problems with reverse popping out with the tuet6400, so tore them both down side by side to try to find where the issue laid.
In the process, i found the following differences (pictures to come, contact me if you want them before i get them posted)
Shift rail bushings
Notched ends of reverse shift-shaft - old 12523202 - new 19133184
5-6 shift selector fork changed to have more clearance to 1/2 fork ?new 19178631?
Front plate bumped out why dunno
1st gear retainer
Mainshaft
Shift guide plate - total forward/rear stroke is 1mm longer. Reverse is 1mm longer as well, with the neutral track being .5mm wider and the top of the reverse track being .5mm closer to the top of the plate (as well as the rest of the gears)
at full throw, the selector for reverse runs into the selector fork for 5/6. Need the newer design 5/6 selector.