http://www.youtube.com/watch?v=xbFyXFtRv0Q
All information on what I did is in the video. The reason why I would call it a “4901-9805 POC (proof of concept)” is since that a real 4901-9805 uses a Faraday horn (a little different than the -9838), and since the -9838 is a rebranded Faraday 6120 horn, I wondered if that horn would fit behind the shell of the -9822, and lo and behold, it does fit nicely (it wasn’t easy, though, but it worked for me).
The 4901 and 4903 housings are all made identical for the different models, the only thing that makes them different are the electronics.
Since the horn assembly from the 2901-9838 is the same size as the one that would be found inside a 4901-9805, it would naturally fit.
BTW, the horn used inside the 4901-9805 and 4903-9219, -9215, and -9217 and their SmartSync versions is actually not the exact same type of horn as the one in the 2901-9838 and its variants. The difference is the one found in the 4901 and 4903 series signals uses a PCB and an infrared light source + a photocell to time the rapid strikes of the hammer, while all the 2901 series horns use a set of copper contacts that move with the horn mechanism.
2901-9838:
4903 series: