Operating the Thornton Pickard, Junior Special Ruby Reflex 1924

I recently obtained this reflex camera: Thornton Pickard, Junior Special Ruby Reflex 1924.
I have a few questions about its operation. Knob A is for lifting the mirror and works OK. Knob D is for setting the shuttertime and can only be moved while pushing knob C downwards. As youn can see the knob B is missing. Trying to turn it with a clamp has no effect, it seems to be blocked (?). What I would like to know is: if you push the mirrorknob (A) should there originally be an automatic release of the shutter? I can only guess what the function of missing knob B is. Is it for winding up the focal plane curtain (which is in raised position at the back)? Hope you can help me. Thanks in advance!
Hans