Out of curiosity I checked on the availability of the TW523 today. I found one listed on Amazon for $200!
Production of that unit was not restored after the X10 factory shut down a couple of years ago, which is why I developed the XTB-523. That unit is designed to be upward compatible with the TW523/PSC05. It delivers a much stronger signal, and provides several additional enhancements.
If you need a TW523 or PSC05, I recommend the XTB-523 as an alternative for anything other than the RCS thermostat. (That combination has one issue that is yet unresolved.)