Thanks Stephen, for confirmation of the rego plate date. I've got some old SCWs from the 63-66 era (but not all), but no reference to my car - like you say, Geoghegans ads were more for new cars than secondhand. They also didn't advertise in the Sydney Morning Herald much, unlike the Alfa dealers Mildrens/Crott/Lightburn and independents like R C Phillips.
I tried the DVLA last year but they need the original UK registration (which I don't have) and said that anyway, their records no longer go back that far, following a transfer of their registry to Swansea. Same with RMS here, their records don't go back that far, even though I do have the EFP164 number. Judging from the amount of rust I'm cutting out of the sills, the car may have been in the UK longer than I first thought. Maybe it was someone's personal import in 1966 (rather than a Mildren import earlier), but that still raises the question of why bring it it in then register it just before trading it in - unless Frank Crott registered (or re-registered) it. I'm not sure I'll ever find out, which is a shame, as that's the only gap in its history.
Cheers
Geoff