It relates to the tritium luminous material used on the dial. Though less so than the originally used radium (dial painters used to lick the brushes they were using to add radium to sharpen the point - see http://en.wikipedia.org/wiki/Radium_Girls ), tritium is still radioactive and concerns had been expressed about the use of the material. Some countries were also applying export and import controls more strictly on the use of the material.

So at some point during the late 60s, Heuer, like many other watch manufacturers, were experimenting with alternatives to tritium lume like LumiNova. These watches don't carry the telltale T, as effectively they have no tritium to declare! This does seem to have been done on a trial basis as we see watches appearing alongside each other with and without Ts.

By about 1970 though, Heuer seems to have settled on an alternative and the Ts more or less stop appearing on the civilian watches. This last point is interesting to note though - the H3 symbol we sometimes see on the Bund military watches denotes the same thing as the T did, that tritium is the active lume agent. And H3 Bunds are still appearing for quite some time after civilian use of tritium has ended. The beta radiation emitted by tritium is fairly weak though and the main danger comes from inhaling or ingesting it, and one has to assume that it wasn't common practice for servicemen to lick or eat their watches so any danger was pretty much trivial!