Just got a private email (many thanks!!) that the part number NDN looks like a Louis Newmark
product. Searching the internet one quickly locates an article in
Flight Global pointing to the
venerable Siddeley Hawk as candidate using the Louis Newmark series 6000 AHRS system.
Cockpit photographs of the early Hawk show knobs for drift/latitude very similar to the ones on the
test set.
So any Hawk / Louis Newmark experts out there?
Other options may be the Sea King or Wessex 3 helicopters according to the message I
got - but the Hawk matches the BFG date of the test set somewhat better...