BOAC:
- I think your picture is not one of those! It looks like a museum piece Narco OBS to me.
I wouldn't argue about its status as a museum piece. Nonetheless, it still meets specs and could be used today. My point in posting it was to show the blue-yellow markings below the course deviation indicator. The "twistable" analog course values at the bottom of the indicator is the selection made by the OBS knob. The OBS selection only affects VOR, not LOC. This indeed is a VOR/LOC navigation indicator with CDI, compliant scaling, OBS selector, and OBS indicator. In more modern displays, such as an HSI, the course selector serves both to position the card and when selected to a VOR facility, it also serves as an OBS.
The use of the blue-yellow indicator was quite common when I first got involved with instrument flying in 1957.
Quite honestly fiddling around to allow for fixed card aircraft is a waste of time! Anyone using it should be able to work out their displacement from a line or should not fly IFR. Otherwise how on earth would they cope with tracking a localiser outbound or track away from an NDB?
Perhaps it was all a waste of time. Unlike you I am open on that. I do know they could not achieve the clearance from very high terrain to the southwest for the missed approach procedures unless they used a localizer.