Thanks for the kind comments re my 12/20's.
Here is what I have been told and managed to discover for my self. After the first world war ended came the great depression which was a very hard time for gun makers. In an effort to keep the workforce fully employed many gun makers started manufacturing guns before an order had been received from a customer. The guns were taken to various stages of completion ready for their final finishing to suit the customers measurements and requirements. The guns I have are the last pair of 12/20's made but never completed by Charles Lancaster. Lancaster's business was bought out by Stephen Grant and the unfinished pair was included in the inventory. Grant finally completed the guns and sold them in 1933. The guns have certain characterisms belonging to both makers like Lancaster's house type engraving but with Grant's fluted fences. The guns have serial numbers belonging to Stephen Grant's register and are cased in a Grant double case. However the guns bear the name of Charles Lancaster.