The Whitworth trade mark is the "Wheat Sheaf"this means the barrels are made from Whitworth fluid compressed steel.[Patented 1865]
Whitworth did manufacture and market shotguns under their own name. I have seen several examples of same,including a 12,G double hammer gun.
It has been suggested that Purdey may have made this gun. The serial number on the gun 6181 fits the Purdey numbering seqence for the year 1861. A call to Purdeys would seem to be justified.

Roy Hebbes