I picked up a nice Purdey 20 bore a couple of years ago from a friend. It has three sets of barrels. The original 29" set and a 28" set added in 1928. Mine was made in 1921 and delivered to it's owner Christmas week of that year. #2 of a pair. He sent it back in 1928 for the second set of barrels and to have a single trigger installed. Purdey's second variant of their single trigger.
Somewhere around that time he had a third set of barrels made, a set of 25" from Westly Richards and so marked with their address and addressing the King at that time. I would only guess for driven grouse as the choking is open in one barrel and tight in the second.
The little Purdey has third bite, clip face as well.
When I asked Purdey about the gun I got all kinds of answers. I think most of the ones you deal with are just salesmen up front. I did get a letter though but the first letter was incorrect so I had to email them to get the correct information on the second email.
Cheers, Mike Proctor