OK Pontius, it all comes back now, you broke it open, bunged in a cartridge and snapped it shut. It must have been the cartridges stowed in the drawer I remember. But the sextant definitely went through holes in the roof, either side behind the pilots' heads.
Now, which aircraft had the flare pistol that could be poked through the roof? On the Whirly the Nav fired the colours of the day at you, straight out of the window. The VC10 didn't deign to fire colours at all, one was supposed to recognise it and bow deeply. You were supposed to recognise the Shack too, but you were expected to giggle a little, while tugging your forelock at it. It must have been the Belfast!
Maybe one of the other old duffers on AH & N can remember, mutter, mutter...