I agree the sextant hangs from a hook in the top of the astrodome. The Astro compass arm was kept in a frame on the starboard side of the plane opposite the astrodome. This fitted on clamps either the port or starboard side of the astrodome. I am trying to make one up so wanted to see an example. The mod to the planes was just a separate astrodome with a flat top and the later mounting where the sextant fits through a hole into the slip stream. The number 1 &2 Air Navigation school aircraft I believe all had a clamp to hold the spare astrodome behind the starboard navigators seat. Also the later sextant fitted in a mounting on the floor on the starboard side just behind the main spar. I have noted the astrocompass mounting in your photo thanks.