Last month when I takled with Hitec the Tx synth module was not showing as a standalone part number. The complete radios however were.
The Tx Synth module will require your older style radio to be sent in for a software upgrade though, as the frequency is changed via software as opposed to a switch like the M8 synth modules.
When buying a new Multiplex, look for a radio that is badged as Ver 2.0