Thanks to Victor Brumby for sending this information about when different the different series of car registration numbers were issued. Victor writes a blog with lots more about the subject, Europlate blog: Registration Plates of the World.
Here's the story for Hong Kong:
Cars and other 4- (or more-) wheeled vehicles
It is unknown when the first Hong Kong vehicle registrations took place. Probably before 1910.
HK used numerals (1-9999) only until 1951, with black on white for private vehicles and white on black for commercials inc. taxis, which vehicles were issued from the same series.
In 1951 they switched to the HK 1000-9999 format. (So 9,000 numerals-only took 40 years!)
In 1957 they started XX 1-9999 (the HK 10,000 took 6 years)
[Update: Several of the commenters below know of XX plates in use as far back as 1954.]
In 1958 they started AA1-9999, AB 1-9999 etc. etc., (the XX 10,000 having taken a year!!) This is the system still in issue.
From 1983, reflective background plates were required by law.
For an unknown period, p.s.v. (buses and hire vehicles) employed white letters on a red ground.
Until 1962, two-wheelers used, first, numerals 1-1000, then A 301-999, then B 1-899, then C 1-899 etc. all in black on white. (1-300 were retained for government vehciles (Administration).
No change to the above systems since 1997.
Any early pictures (say pre-1955) showing plates, I'd be delighted to see....
Your street shot of the new-looking Austin A70 Hampshire would have taken in 1951 or 1952, I opine.
Thanks again to Victor for his help,
PS Gwulo has a collection of photos tagged vehicle registration, ie photos that clearly show the plates and their registration numbers.