Help with a muff derringer

[t3tikkat3]

Can anybody help me with these pictures.My neighbour had this from a long time ago. Looking for help about it .I believe it was what was called a muff pistol but no quite sure.The bore appears to be .22 cal and is a smooth bore but with the corrosion and such I am not quite sure here are some pictures .any help would be greatly appreciated...

Thanx in advance... t3tikkat3

Also can any one tell me what the proof marks might mean...

 

[icehunter121]

Must be a pretty small muff to hide that one..or really tight