I bought a second-hand X50 with a build date of 3507 with a screen that was blank (and the mute button was missing). I sent it in to Escort for repair. I got it back and today I checked the version and it was a Rev 5! (the serial number sticker shows "M4")
I really don't understand, I thought it would be in the 6's. It did come with a note saying it was repaired 4107, so maybe there was a board switch at that time?
I'm not complaining about it, I already have 2 other Rev 5s, one blue, one red. I kinda wanted one in the 6s for comparison's sake.
I checked this thread, and it didn't clear it up:
Escort 8500 X50 - Software Version?