Rex, sorry but have to disagree ,unless DVLA as seriously changed /improved it's staff . When i took my bike to be checked over the "inspector" was a young kid who didn't know one end of a bike from the other ,He started by calling my bike a Bantam , I had to show him the frame number which He looked at for a nano second , He was much more interested in the receipts which I had in spades . I have to say this was 11 yrs ago but thatwas how it was done then.
Things have changed a lot. My last one was back in the late Spring, and the inspector was a woman, and an employee of the firm the DVLA have sub-contracted to do inspections at owners homes.
She either knew, or had done some research before her visit, but she knew what she was looking for and what the bike ought to look like.
This-
Do not give the DVLA the chance to screw you over. Stamp the frame to match. Seriously, really. Any other route is going to cause you pain.
is a little puzzling. In stamping the frame you would be giving the DVLA every opportunity to "cause you pain", unless you are a pretty good forger and have a set of BSA original font stamps. A set of machine mart 6mm metric stamps aren't going to fool anyone, especially those paid to spot recent restampings. Apparently, DVLA's approach is "restamps are iffy so we'll look a little deeper".
It could be someone who's circumstances are entirely innocent (as yours) but more likely it could be a dodgy number seller, a stolen frame/bike being given a new identity, an A10 into an RGS, etc etc none of which will impress the DVLA.