Chugalug (your #7549 and #7554),
Fareastdriver (your #7551),
bingofuel (your #7552),
topgas (your #7553), and
BEagle (your #7555),
Thank you all for your helpful suggestions and comments on this prize example of double-ball-itis. What has emerged is, I think, the right answer: the additional ball (below a Directional Instrument) formed part of a Sperry autopilot control sub-panel. Here I am at a disadvantage, having never seen (still less used) one and am indebted to Wiki for this pic of a modern one.
The modern flight control unit of an Airbus A340
But in earlier times (can't find a pic) the auto control panel seems to have been found a place in the pilots' main panel (as in this 1938 Lockheed Super Electra), and I would suppose the wartime DC-3s and Dakotas would have a similar fit (we have wartime Dak operators on frequency - come in, please).
Clearly you would need to be able to fly the aircraft with or without the autopilot fitted, so you would also have the normal instrumentation. Indeed the Electra has another AH on the Captain's panel and surely another DI (blanked off by him), as well as the needle&ball up in the middle (odd place to put it ?)
So problem solved ? Well, no, not quite, In training in the US, I flew PT-17, BT-13 and AT-6A. Back home I flew Master, Hurricane, Spitfire, Vultee Vengeance, Thunderbolt, Harvard again, Spitfires again, Meteor, Vampire and a Balliol. None had more than one ball or slip needle.
So tell me why the P.40 had two (no autopilot). Were any other US s/e types so fitted ?.
There were some interesting sidelines:
The Captain had to make do with two stripes, the F/O only one (Imperial Airways ?)
Chugalug: I'm sorry I gave you triple vision, but, honest Guv, it wasn't me ! I managed to copy the picture all right, tried to paste it on PPRuNe-pad three ways, no joy on two of them and the third just gave me an 'url' (whatever that is) reference. Gave up, just put text in, "Preview Post" showed just text, tapped "Submit" - and all three chickens came home to roost ! Sighed and went to bed.
Yes, some instrument layouts were compared with "Pawnbroker's Shop Windows". Ju388 is new on me, had to look it up on Wiki. Good performance. Vertical scan might be easier than horizontal ? Turn indicator might be that little clock at the bottom, but how would it work ?
Fareastdriver, "...Would it be possible that the 'ball' is an ADF indicator of some type..." Don't think so. Will hand that over to those far more knowledgeable than I.
bingofuel:
"Is the large rectangular group of instruments not the basic autopilot fitted to the ,...." Spot on !
topgas: "...even his 1250 are included..." How on Earth did he manage to get away with
that ? 'Fraid I dont know Mepal or 75 (NZ) Sqdn, but met many Kiwis in my time. Always thought that, of all the Wild Colonial Boys, they resembled us the most closely.
BEagle: Thank you for a much better picture ! - but:
"...and the other is a straightforward back-up..." I would have thought that, of all things, a 'ball' slip indicator was the least likely to fail. "
Gravity never lets up", said one of my instructors in the US.
Once again salutations and my thanks to you all,
Danny.