Martyn, on the steel wheeled cars the drums are located by the spigotting diameters of the wheel studs. Theses are the same PCD/size (nominally 7/8" outside diameter)as the slotted screws on the wire wheel cars.
The rear drums are identical whether on the 4 stud cars or wire wheel and Lockheed (>93) or Girling (<93) and have been since the change to 1 3/4" width. The 5 stud drums locate on the studs, and have a single countersunk retaining screw
Simon @ SiFab.co.uk
True the location in use is given by the studs and the two screws are merely to hold the drum in place while wheel changing. As the screws are countersunk you cannot remove the drum with them in place. Undoing those is the OP's problem unless I'm reading what he has said all wrongly.
