Thank you to all who contributed to answering my question.

pjb

: 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!