Len,
During the years of production of the Mks 3-5 the battery polarity did get changed, but the polarity has no effect on the wiring diagram - it's the same for positive or negative earth. Most Mark series Douglases have the earth side of the battery connected to one of the mounting bolts for the battery carrier. The live side of the battery should be connected to the ammeter, and the other side of the ammeter to the lighting switch. If these wires get reversed, the ammeter will show a charge when it should show a discharge, and vise versa. If this happens just swap the ammeter wires over again! The horn puts a greater drain on the battery than anything else, so ideally should have the wires as short as possible - the wiring diagram shows the feed being taken from the ammeter, but it is better to take a second wire from the live side of the battery to the horn (it's mounted quite close to the battery). There didn't use to be any problems with bad earthing prior to the advent of 2 pack paint and powder coating - nowadays, it's good practice to install an earth link between the engine unit and the frame to eliminate any problems.
The ignition cutout is simply a wire from a horn type button on the handlebars to the points cover on the back of the magneto - pressing the button shorts out the spark!
Eddie.