Ranfft is more of a guide than a comprehensive catalog. The 27M can be found with either 15 or 17 jewels, as with most of Longines' time-only calibers. Cal. 27.O ("O" standing for "or") was reserved for solid gold cases and therefore had higher end features like the Breguet overcoil hairspring and 17 jewels.
Oh well, so much for my idea. I'm not too surprised. It's pretty common to see variations of the same movement with different jewel counts, depending on when it was made, and to where it was exported. Roland's archive is an extraordinary piece of work, but it doesn't necessarily have every variation.
@Maskelyne - I wish I could look at it right now and measure the diameter. It is in a wardrobe in my parents house in Melbourne. I am in Los Angeles until....
I promise I will get a picture up as soon as I can.
I never thought any of these Longines watches would ever be worth anything.
