The original position was the high voltage nipple on the coil pointing down. Apparently, the idea was to prevent moisture, condensation, or rain splash from shorting out the high voltage spark. The coil will work the same if it's up, down, sideways, or backwards. It doesn't know, or care what position it's in, or where it's mounted, including on the tailgate!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!