A Transitional California Style Holster ("Slim Jim") with a single Laced "Mexican Loop".  
Suited to percussion revolvers and early cartridge revolvers with longer (7 to 8 inch) barrels.
Inspired by my memory of the holsters worn by "Hipshot Percussion", in the "Rick O'Shay" Comic Strip (written by Stan Lynde from 1958 to 1977).