Colin,
This will have stemmed from when the DVLA went computerised and I'm sure that the following information is reasonably accurate.
In the late 1970's/early 1980's everybody had to return the old green or buff logbooks to have the data transferred to the then new DVLA computer. Initially we had about 18 months to do this but there were literally thousands of vehicles in storage and the owners either weren't interested or couldn't be bothered to send the old log books in. After the deadline and after getting loads of complaints from these owners who didn't comply, the DVLA extended the deadline to about an additional 18 months. These late ones were actually re-registered on the DVLA computer but a note was added to the new V5 documents that the vehicle was actually manufactured in **** whatever year.
There was an extra problem after the second deadline expired and older vehicles were still coming out of the woodwork. These were not registered and many of the owners didn't even have one of the older log books. It was decided that these could be registered by DVLA but would obtain either a "Q" registration number or an "A" number (e.g. ABC 123A) depending on certain circumstances but the owners had to prove the year of manufacture and that they were genuine vehicles. At this time the "Q" plates were generally issued to imported vehicles and "specials" such as Tritons, Tribsas Norvins and the likes.
Some time later the rules changed yet again as some owners weren't happy with these "A" numbers and DVLA allowed them to be changed to an age related number, I believe for a fee. This was when a series of unused Scottish numbers was issued. These were instantly recognisable because the second letter was an "S" and we saw the likes of HSV 186 or SSK 111. I think that newly re-registered vehicles are getting local older style registration numbers now.
So check your V5C to see if there is a "Declared date of manufacture" on there somewhere. It will be in a different place to the date of registration. Mine is shown in section 3. under "Special Notes".
If there is no mention of it I would suggest that you wrote back to DVLA and ask what the procedure is for this information to be added. You could also phone them up but if you do, make a note of the time and date and the name of the person you spoke to.
Hope this helps.
Beezageezauk.