Interesting...
That explains why I've never seen one of those loudspeakers with the four terminals - Canadian only. (Likewise the C.U.190.)
It's also going to be Far Too LOUD for your intended application: the intercom amplifier is a 6V6 and designed to drive up to 7 headsets (possibly more), and matching the impedance for full output is what you would want for a 25 pdr crew listening to fire orders.
Likewise, it will be too much load for the poor old 'A' set output stage (a 6B8 driving a transformer that's a fraction of the size of the one in the Tannoy speaker).
I think you will need some fixed resistance (probably around 70 - 100 ohms though that's guesswork at the moment), and a variable one (as volume control) in series with the "WS19" terminals, and connect it to a droplead or direct to one of the terminal strips in a control unit.
Which control unit(s) are you using with your set?
Best regards,
Chris.
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