Looking at the table in the instruction book, it uses X synchronization at all speeds.
This was something of a handicap in its day, as the overwhelming majority of people using a Signet 50 would be using bulb flash. With bulb flash, one would be limited to the 1/30 s speed (at faster speeds, the flashbulb would provide good luminosity only after the shutter closed).
Some other cameras (I think my Argus A-four is one, though I'm not sure) had a better system (for that day) - It had M sync for 1/200, 1/100 and 1/50, but X synchronization at 1/25. X at 1/25 both allowed electronic flash and getting all of the light from the flashbulb, whilst the M sync at higher speeds allowed bulb flash of action scenes with some control of motion blur (one could use Tri-X at 1/200 and a #25 to get a sharp picture at a basketball game).
Am I the only one who thinks it is odd that they mention that the Signet 50 syncs electronic flash at all speeds, while making no mention how to attach an electronic flash to the camera?