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Rolex

This gives some guide line but not real explination

 

I seem the order is exclamation followed by underline with the overlap in the middle.

so we could say the most logical time line is:

Exclamation
1961-1963 with a serial range 600,xxx - 990,xxx

Underline
1962-1963
 +/- 800,xxx and upto the end of the pointed crown guard +/- 1,1xx,xxx

Not an exact science for sure.  This does seem to match the gmt and subs in that period.

Bill

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