Bas's 6610 below is very interesting to me:
Why is this 6610 dial marked T SWISS T? Could it be an early service dial, as this is certainly an atypical notation on a 6610, AFAIK, a model reference that predates the "T" marking era by several years?
It got me wondering if this type of "Round Bottom" Coronet Explorer dial, which we see right up until the end of the minute track era, might itself indicate a "new lume" dial, that is to say, a replacement and/or one with reapplied or tested-safe luminous.
For example, some more Explorers with extremely similar Coronet & top dial text:
5504 SCOC w/Underline posted by Marcello:
1016 Underline, 915,k SN & II.63
1016 Underline and Ex Point, 9xx,k SN
(posted by Terence Ho in the Archive):
Another 6610 without T markings
and definitely different type of lume (posted by Dab from the Archive) with this very interesting notation:
"This Explorer came with box + guarantee dated 5-2-60 and a chronometer certificate dated September 21, 1957"
i.e. sent for Chronometer testing in the late 50s but not actually sold
until the middle of 1960 (plenty of time to have the dial pulled and/or relumed).
And now Bas's 6610 with neither Underline or Ex Point and with different OCC but same
Coronet & marked T for Tritum!
Maybe Rolex were using up old 6610 dials for the "new" 1016? But that still wouldn't explain "T SWISS T" here which, aside from not being found on 6610 dials is not reallly found on Explorer I's either, i.e. it is invariably "SWISS T-25." T SWISS T would more likely be an early prototype marking or
the Ts were added after the rest of the dial had been printed.
Could it be time to bust out that Geiger counter again, Bas & the Dutch Boys?