Salut Philippe,
I´ve nearly the same movement / watch from one of the founder of MAGNETI MORELLI (that´s what the seller told me)!. Till today I´m walking in darkness, who made this movement kind. The seller from which I´ve bought this watch some years ago mentioned a kind of prototype, but I don´t think so. Even after I´ve observed two more watches from the same kind here in this forum.
In my books I estimate the "M" for MOERIS… Probably it might be a rare MOERIS chronographe during the time, pocket watches moved to wrist watches. My diameter is as you mentioned before approx.16 1/2 "’, too.
Cordialements
Antoine