I've occasionally seen old British bikes with reversed polarities on the dynamos ( generators ). It's just a historic fluke that results from incorrect reassembly procedures at some time in the past. Ammeters were usually the first part of the system to give up and few riders bothered with them on dynamo equipped bikes once broken, you knew if your dynamo wasn't charging because the lights would fade after an hour or so. The usual mag plus dynamo, or magdyno system meant that the bike would run quite happily with a flat battery or no battery at all.
Manuals would often contain this sort of thing http://www.mgccq.org.au/tech8.htm
.. it matters more for cars because the starter motor has to run "right way round" whereas a bike system will run quite happily on either polarity. Negative earth ( ground ) systems were generally considered more reliable but whether that is actually true, I don't really know. The main thing was to know which it was!
You don't see them these days because the days when running bikes were flogged into the ground, stuck in sheds, forgotten and dragged out again years later are long gone; pretty much any bike with that sort of electrical system will have been rebuilt at least once and problems of that sort eliminated.
Zener diodes, or the flat-plate rectifiers which preceded them, relate to the later alternator systems. Dynamo equipped bikes would have a contact-breaker type voltage regulator much like the H-D one, usually mounted on the rear mudguard somewhere under the saddle.
Shoot, a man could have a good weekend in Dallas with all that stuff...