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).
Serial Number Range: 62xxxx-990xxx
Serial Number Range: 687xxx-11xxxxx