Confirm that you are talking about a Purdey best sidelock. They were selling some graded SL's and boxlocks at that time. Ejectors were rapidly becoming a common feature then, but not entirely established at that point in time and not definitive of a best gun. Also, confirm that the gun is a game weight gun (I believe that is what you want) and not a pigeon/fowler.

Remember that the brand name is not definitive of the original quality of the gun. Brand and quality are separate items.