Omega Pocket Watch identification

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Hello everybody,

Today I got this pocket watch from a flea market: as you can see it's in pretty rough shape, but what struck my attention immediately was the size of it
I measured It once I got home and it has a diameter of 57mm, the case is signed Omega on the inside of the back cover (also, there seems to be a "6" and a "D" on both the dust covers, what could those mean?)

I decided to buy it given the very low price (30€) and the movement which seemed to me in good condition (the balance moves freely but I think the mainspring is either broke or missing, since turning the crown does not wind the watch)

I used some tools online to try and date it and it seems like it's from around 1916, but this is all I could get

I would like to know some more about it, especially about the size since I hadn't seen one this big before I bought it (was it common for pocket watches to be this big?)
Also if you know what movement this is I'm all ears!!

Thank you all in advance!