General Notes: Many have speculated about what the underline and/or exclamation point signify. The commonly accepted theory is that they demarcate the transition from high radium content to lower radium content and possibly the start of tritium use before the “T” designations became standardized in 1964. The exclamation point dials still have some Geiger output, though lower than earlier 1675s, and underline dials usually have minimal, if any Geiger readings. This also corresponds to the slightly later serial range of underlines compared to exclamation points. Here is a pair of articles by Rolex Passion Report on Rolex luminescence (here and here).
Exclamation Points
Serial Number Range: 62xxxx-990xxx
Chapter ring dials that are seen with exclamation points: Type 2 (more common), Type 3
Underlines
Serial Number Range: 687xxx-11xxxxx*
Chapter ring dials that can be seen with underlines: Type 2, Type 3
Non-chapter ring dials that can be seen with underlines: Type A, Type B
*this serial range is the total range that dials are seen with underlines – this does not mean every dial type with an underline can have any serial in this range. Look at the individual dial pages to see which serial range is correct for each dial.
Rolex GMT Master 1675 