I recently acquired a very nice late gilt 1675 GMT from the son of the original owner with a lesser discussed variation seen on some mid-1960s GMTs and Subs: bitonal lume. For some reason, around the 1.4-1.6m serial range, the lume on some examples is two shades -- darker on the circular lume plots and lighter on the 6/9/12. There are some late 1950s watches that have a lighter colored 6 o'clock plot due to their different radium content, but this is the tritium era so there shouldn't be any radiation effects. Might be due to the manufacturing process (maybe different machines applied different plots and the mixtures were different) or a difference in aging (more surface area to volume in those plots --> lighter patina), but all speculation. There are also Bart Simpson and early matte meters first dials with this same variation, all in the same serial range.

Nonetheless, I like the subtle difference. If you come across one or own one, I would be interested to see others.


(Also, thanks to @leefowler for selling me the small GMT hand to correct the service replacement that is seen in this picture)

 

Oh no! Another knot in the lash!

 

This has been discussed on VRF though it is usually the older dials, like you said.

I think it is the manufacturing process. Look at how near perfect the round plots are and how the long plots have (self) altered. Too bad we do not have super macros from these watches when new ; ) . I suspect the long plots have shrunk naturally over time. In the entire sixties and much of the seventies, they were constantly changing lume compounds and application techniques.

Fantastic dial!