Hi Paul,

Thanks for your most impressive analysis. FYI, the "T" stands for tritium, the lume that was used until approx. the end of the sixties. Tritium is slightly radioactive, therefore the "T warning"on the dial. I own a Blancpain Fifty Fathoms from 1968/1969. The first FF without tritium. Blancpain was so proud that the dial contained a huge yellow circle with a crossed radioactive logo ( the fan or turbine symbol) and the text "no radiation". Heuer may have switched around the same time period.

By the way, I see one more difference. The luminated dots at 3, 6 and 8 o'clock are placed a bit more inward in the later version.That's obviously a result of the longer red markers.

Cheers and enjoy the weather! ( 22 C in Holland tomorrow!)
Ron