Ya know what's kinda funny here guys is that when I just Wiki-ed "Mustang(motorcycles)", there's another pic of a Mustangs with a Triumph Twin engine right up at the upper right of the page. And so, maybe this wasn't such a rare custom-bult combination after all?!

http://en.wikipedia.org/wiki/Mustang_%28motorcycle%29

Yep! Just like a good Single Malt Scotch, you might call me "an acquired taste" TOO.(among the many OTHER things you may care to call me, of course)