Perhaps it is easier with an example.
Let's say that Omega produced 10 watches of the 2913, 2915, 2914 series each year from 1955 to 1961.
Most of these had the smaller crown of the naiad type ( 10 in the first year, 7-8 the last year)
Some of these came from a certain maker who used also the Y symbol.
Best wishes
Franco