IMHO it may be a post-war production astro compass Mk. IIA but not Mk.11A. I don't remind myself when the Air Ministry changed the Ref. numbers but as you may notice the no.1174 belonged to the 6A/ prefixed accessories while the one you posted above to the 6B/ one. However, if memory serves ,I have seen the 6B/ prefix on a Canadian made astro compass Mk.II of 1942. According to the A.M. list of the Ref. numbers there were the ones ...
6A - Aircraft Engine and Flying Instruments, Accessories and Spares
6B - Aircraft Navigation Equipment, Accessories and Spares
So .... the astro compas of the Ref.No. 6A/1174 belonged to the Flying instruments what allow to believe it was a fixed device used on the aircraft deck as the pilot/navigator's instrument. The 6B prefix may suggest it wasn't the fixed tool . However the astro compasses weren't fixed there mostly but their bases only. The device was kept in a wooden box usually ( the ones made in the USA in the bakerlite boxes usually ) and taken out of the box when needed only. As I mentioned it above the Air Ministry could have changed the prefix of the accessories and its number for the devices after the war. As a result the 6B/399 may be nothing more but just the 6A/1174 or 6A/1174O. Also it should be taken into consideration the the astro compass Mk. II and Mk.IIA were made not only in the GB but also in Canada and USA by a couple of manufacturers. Looking at your device I would say it is closer to the Mk.IIA 6A/1174O judging by the sahape of the shadow bar that is in the shape of a cross rendering the instrument more sensitive in low latitudes. Because the devices were used for many planes it is impossible to state any particular one.
