B737- 400 Unknown Switch
 

https://cimg0.ibsrv.net/gimg/pprune....3f97e01d3f.jpg
B737-400 half glass, I cannot find a reference to this switch anywhere, FCOM 1, 2, FCTM anywhere. To the right and left on MCP panel, it where the nav/hsi switch is on steam 400's.

A customer option on the Classics provides for dual Alternative Navigation Systems (two inertial reference systems independent of the FMS). As an option, it won't appear on all aircraft, and presumably perhaps not in all manuals.

Navigation

We had this on our -400s. Do you have a serial number for the aircraft in question? You can pm me if you like, I still have the old manuals tucked away, probably not more than a page or two to scan.

Saw this on a -300 many years ago. It is a back up nav system and you can read more about it here-
Navigation

The mighty ANCDU!

With a storage of 20 waypoints, this was a sort of evolution of the CIVA or Carousel navigation system that was available on the 737-200 and 747-100/200/300 and it could hold only 9 waypoints if I am not wrong. We have two -400 with this option in our fleet.

oceancrosser

I'll see if I can get you a serial number next time I'm on this bird.

Think they were popular with the Ansett B737 flying around Australia with not many Nav aids and pre-GPS.

ANCDU Tech manual extract
 

A former airline had some former Ansett aircraft. Attached is the extract from the Tech manual.