: So I wonder whether yours marks the transition from radium to
: tritium? Or perhaps a transition in the requirement to have this
: "T" marked on the dial?

It's possible the earliest Autavias used radium, but fairly unlikely - some watch manufacturers were still using it in the 60s, but mainly for applications where "extra" luminosity was required like dive watches and some military applications needing good night readability. It was considered not entirely ethical at that point too, given that the radium girls case had flagged up the dangers of radium as early as the 1920s. I think it's more likely that it ties in with the recognition that tritium can be dangerous as well (though only when ingested) and additional legislation coming in regarding its use and export controls. So my best guess would be that both T'd and T-less watches use tritium, but the later watches are obliged to notify this on the dial. This could be a very good question to pose to the ex-Heuer watchmakers I reckon.

: It would be great to receive more photos and serial numbers, and I
: will undertake to update the Registry. Perhaps we can also
: detect this movement from "Swiss" to "T
: Swiss" in the early Carraeras?

We can indeed. Some of the early polygon-caseback Carreras have no 'T' on the dial either, on watches with no dial retouching. So that would put the phasing in of that legislation at some point during 1964 and trials of other lume materials a few years later (for Heuer, others had started earlier), which makes some kind of sense.

The 1962 catalogue that introduced the Autavias:

doesn't show any Ts but we can't always trust the catalogue shots for the fine details. I really like these large-register Autavias - Peter had a splendid two-reg example at the auction.