Here is one the has me stumped. It appears to be a 1950's model but it clearly has a M4 date code. The case is a 10KT yellow rolled gold plate case and case back. The case back has an M4 date code. The dial is silver with applied gold markers in 12, 3, 9 & 6 numbers. The movement is a 10BP with 23 Jewels but with an L9 date code. I've searched most of the line books from 1955-1965 and can't seem to find this case. The case back appears to be an exact fit so I do not think it has been swapped. I cannot explain the 1959 movement date vs the 1964 case date. Everything looks factory correct.
An attractive watch that does indeed seem out of place in the style timeline. I have come back to this one multiple times and can't find a match either. I wonder if this could possibly be a masterful combination of parts. Sorry, man.
Unknown for now; noting gap in dates.