My guess is that the FSB is an improved PSR rather than a cut down FSX. But the name implies the latter and the description has a lot of features in common with the FSX.

Having disassembled both FSX and PSR Keybeds the main difference is a discrete metal spring and some weight on each key for the FSX . The aftertouch mechanism does not affect the keybed feel. The FSB has gained some weights and that could mean a spring is necessary because the very crafty but inexpensive use of the rubber contact strip as the spring mechanism on the PSR might not be strong enough.

Anyone seen a service manual? That would give some good clues!