This is the first Purdey I have seen with this feature. I'm sure it probably has some sort of camming lock. [not the easiest thing to cut into an action] If you look closely at the photos, you will see a tiny arrow showing the direction of rotation of the thumbpiece engraved on the lockplates. I would have to say that they look original. If you are willing to pay, almost anything can be done.

All the best, Mal