I'm fairly sure the 190 was a Canadian 'special', as they also seem to have had the Tannoy gun control loudspeakers with four terminals - two marked "Line" and the other two "WS19".
My guess is that the WS19 pair go to different taps on the 100 volt line matching transformer (used for Apparatus Loudspeaking) and provide a headset-equivalent impedance that won't overload the set output transformer, but there's always a chance that the C.U.190 uses the IC amplifer to boost the output (and that the extra resistors around the top switch are to provide a two position volume control).
It may in any case provide far too much output to be used as a monitor speaker.
Best regards,
Chris.