What is with the extra ground wire though. Should we be using a 3 wire one. 2 wire was what was in there, 3rd wire hanging.
The original flashers on the US market bikes were 2 terminal devices and the 3rd (green - ground) wire was just left hanging there. If you look at the wiring diagrams for other markets, it was sometimes used. I have no idea what the requirement or functional differences might be, between the flashers supplied to us and some other parts of the world - mostly Europe.
I usually use the auto store 2 wire (so-called heavy duty or towing) flashers, mostly because I have a small stock of them from the old days when I owned a shop which installed trailer hitches.
The third wire is certainly useful if you want to go with an electronic flasher, which is something I've been thinking about. I just haven't bothered with it because my personal situation is not allowing me very much free time recently.