Dennis will probably know the answer to this, but let's see how many others will be successful. Based only on the photo hint below, what is the model that originally housed the movement?
No, sorry. Bear in mind that I focused on the regulator for a reason – it's the 'tell'. The base movement is very well-known.
That's a Cal 30L, a version of which was a worthy opponent of the Zenith Cal 135 in observatory chronometer competitions. I think this is the observatory version. Want one.
Here's my 30L: The 30L (subdial seconds) and 30LS (center sweep seconds) were first produced in 1955 to go inside the Flagship models.
Ah, now we're making progress! I'm afraid that it's not quite as rare as the Observatory examples of the 30L, but it is from a Flagship Chronometer. I found mine at an esoteric auction in the U.K. last year, and it has just returned from my watchmaker. I haven't taken a good photo of the dial yet, but will post one soon. There are a few other distinctions between a regular 30L movement and the Chronometer version, but the truncated regulator tail – needed in order to avoid contact with the Breguet overcoil hairspring (not found on the regular version) – is the most obvious difference. Here's an old French advert that shows the Chronometer version. You can see the word "Chronometer" printed under the Longines wings.
I can't believe I finally get to say this to you: you're on the list of members who owe pictures. We like any Longines that's chronometer rated. A Flagship Chronometer though....... let me just say "DIBS!". I'm still kicking myself for not flipping a few watches to make a run at that chronometer rated 27OS you sold a few years ago. That was a peach! http://www.watchtalkforums.info/forums/longines-forum/28270.htm My U-C Chronometer caliber 6651 paired with my 18ct U-C caliber 431: