I would contact Purdey with the serial number for information on the gun. If rebarrled by Purdey they will tell you the guns history. I would have a person who knows British shotguns evaluate your Purdey.
As to the Model 21 and Model 12. The Model 21 is an early Model and its probably done by the person that did the Model 12 work, if it is Paul Yeager as you indicate. Your going to have to find a Paul Yaeger fan to sell those guns.-Dick