Interesting one on the retention plates - never realised you could keep the original age-related reg "in reserve" like that while running a cherished plate for looks. Seems like a sensible way to have your cake and eat it, appearance-wise vs provenance-wise.
On the DVLA point, my understanding is they'll only issue a period-correct age-related plate if you can prove the car was actually in the UK/on these shores during that period - reflects the import date, not the manufacture date, unless you've got solid evidence otherwise (old logbooks, dealer invoices etc). Worth a shot if anyone's got paperwork going back far enough, but for most imported cars the Y/T/whatever reg is just going to reflect when it landed here rather than 1969 or 1973.