I'm still digging into the history on the car, and have an interesting possible twist (or alternatively nothing), I'll share here on the outside chance someone reading it may have a tip for further research.
From Andy Graham at Lotus and John Watson, the HLR 7 Registrar, my car left Cheshunt in 1965 - and its unit number (SB2042) fits for mid-65. The frame number, engine number, and carb numbers on the car today all match the Lotus records for SB2042, and the pictures and records I have pieced together from over the years convince me this is the same car recorded in the logs.
According to those logs, the car was sold to a Mr. Slatter. A couple of the Australian owners since have said that Slatter was an Australian national who lived in London and who worked at Lotus at the time he purchased the car, and who took the car to Melbourne, Victoria, Australia when he emigrated back in 1965 or 1966. (I've found an online record of a Slatter (John D) who was a metallurgist that relocated from London to Melbourne in late 1966, who may have been the same person.)
Until now, an oddity in the history is that a registration document indicates the car is a 1964, and that is how its various Australian owners have referred to it. The running theory has been that it was either an error or that the owner claimed it a 1964 to avoid being taxed as a new car on import.
Pursuant to an FOI request, I've received a copy of the original application for registration of the car in Victoria (unfortunately with name and address redacted, as is the custom). According to this application SB 2042 (with matching information to the Lotus logs referenced above) was registered in January 1967.
An unexpected finding on the application is the previous registration number, which as it turns out is a registration number from England (picture below). Unfortunately the last 2 digits are unclear, but I've asked if vicRoads can identify a clearer copy.
What I can read is the first five digits, KAR96. The last digit could be a C as one would expect from a 1965, but also looks a bit like a B as one would expect from a 1964.
[ATTACH=CONFIG]16520[/ATTACH]
I understand the AR in KAR indicates the car was originally registered in Hertfordshire County (Cheshunt?). Can anyone here confirm that is accurate?
If that is the case, I wonder if the following is a possibility: could the car have been a factory or test car of some sort, registered originally by Lotus in 1964 (with a "B" suffix), known to Slatter and eventually sold to him in 1965, at which point it got its unit number and entered in the logs?
That all seems highly unlikely to me, but enough pieces fit I thought it an interesting possibility/theory on why the car was eventually registered in Australia as a 1964.