X2^^^^^
It's just wrong. I would seriously doubt any markings on the barrels, if indeed the stamped numbers match, as it sounds like a back-door cobble and restamp job to me.
If you're in doubt, call Purdey, I'm sure they would be more than happy to help. I personally haven't dealt with them, but Holland & Holland, and EJ Churchill have been very helpful to me in the past, and I would expect no less from Purdey.