I suspect the Percy Stanbury gun was a Model 600 Special (with ejectors) just like my one, which was sold in December 1946 but probably started life before 1939. It has 30 inch barrels and a semi-pistol grip, but two triggers.
I have cleaned it and put it away after shooting 3 duck (with steel) and eight pheasant (one ounce of lead) on our local driven shoot. They still work...

My friends Greener played up so I let him use the Webley - three birds for four shots.

HB