Dating Movements by Cyma & Tavannes

I recently had a query on dating these movements so have posted the answer here so I'll not have to do it again.

The serial number on these movements are under the dial as show to the right. My dating is based on a reasonable number of watches, listed below, in hallmarked silver or gold movements. The sample is heavily biased to watches retailed by J.W. Benson, largely because other retailers frequently used Swiss hallmarked silver cases which do not carry a date mark. Benson appear to have switched from Revue to Tavannes in about 1932 the database is only good for the 1930s.

It must be remembered that a movement might not go straight into a case which itself may have been on the shelf for a while so there will be some overlap, also the hallmarking years of the various assay houses are not in line with each other or the with calendar but the difference is small and the date is likely to be accurate to plus or minus a year.  (updated Feb 2017)

Serial	Date
4,322,549	1908
12,487,883	1919
14,708,666	1919
13,079,475	1921
14,262,246	1924
15,485,579	1926?
15,485,952	1926?
15,939,146	1926
17,482,498	1929?
16,624,975	1930?
17,865,828	1932
17,886,973	1932
17,888,724	1932
17,888,811	1931
17,889,975	1932
17,899,152	1931
17,899,219	1932
17,889,918	1932
17,902,501	1932
17,903,525	1932
17,903,625	1932
17,908,930	1932
17,909,034	1932
17,909,035	1932
17,944,833	1933
17,944,871	1933
17,966,044	1933
17,970,131	1933
18,045,354	1935
18,045,567	1935
18,050,727	1935
18,052,802	1936
18,055,283	1936
18,055,497	1936
18,055,498	1936
18,055,544	1936
18,066,750	1935
18,070,392	1936
18,076,649	1936
18,090,883	1936
18,112,605	1937
18,112,379	1936
18,113,219	1937
18,141,556	1937
18,146,434	1937
18,146,664	1938
18,199,372	1937
18,240,750	1938
18,245,978	1938
18,246,001	1938
18,252,273	1940	Benson stock?
18,253,220	1939
18,256,774	1938
18,265,158	1938
18,268,000	1940
18,276,566	1947?