If we look no further than the OTD galleries we find different samples of early Carreras without the T.
Here's the links - the "no Ts" are at the beginning of the galleries:

http://www.onthedash.com/Guide/_Chronographs/63.Carrera/a.Manual_(1960s)/Carrera_12/a.Reference_2447S_(Standard)/

http://www.onthedash.com/Guide/_Chronographs/63.Carrera/a.Manual_(1960s)/Carrera_12/e.Reference_2447N_(Black)/

: Indeed, we have no "T"s in our registry of the early
: Autavias -- http://www.onthedash.com/docs/AutaviaRegistry.html

: 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 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?

: Jeff

: +++++++++++++++++++++++++++++++++

:
: :
: :