I was interested in getting one when I owned a GL1500. I have seen a handful online over the years. I would classify them as uncommon but with a low demand as well. Just a matter of finding someone who wants it more than you.
As to history, for those where I was able, I traced them back only to Canadian dealers. As far as I could tell, they were a sales incentive to help move some languishing inventory. I may well be wrong here but none that I found were ever sold by US dealers.