Maybe boats were measured with the inside dial, and planes were measured using the outer. This would explain the RAF clock only having the outside color markings.

I've been doing web searches for Sector Clocks off an on since this topic came up and so far all that I can find are two styles - the RAF ones with RAF markings on the dials, and the Air Ministry ones with the AM markings on the dials. They all have just the outer markings. These may very well be some other kind of Sector Clock but what keeps me from reserving judgment so far is that these two examples we have seen now do not have any military markings of any kind on them, and I have not yet seen any photos of the WW II or post WW II era where you can see one of these - unlike the normal RAF Sector Clocks. They may very well be UK RAF, Navy, or Army Sector Clocks of that general era (post war maybe?) but I would really like to see some specific reference or period photo or one of these with military markings on it.

Well, For a small fee, Chelsea Clock Co. will provide a certificate for the particular serial numbered clock with the purchaser and date purchased. That might be the clue we need to solve this boggle..

Here is what was going on with aviation during the time period that the Chelsea was produced:
http://history.nasa.gov/Timeline/1950-54.html

It is a Chelsea Sector Clock.

The military switched movements pretty often.

You can contact Chelsea and the number scratched on the back of the case will for a fee identify who originally purchased it.