Any pictures of the proof marks on the action flats? Do they corroborate the barrel marks?
The seller has kindly sent a pic of the action flats. The stamps do corroborate the barrel proof marks. There is no "Quality E" stamp on the action flats either, indicating this is not a lessor quality gun built by someone else for Purdey with Purdey putting their name on it, but rather this was/is the real McCoy.

Still awaiting Purdey's response in an effort to see what information they can provide and whether they're original Purdey barrels or someone else's.