As mentioned, the fork is specific to the frame, as the steerer is much longer. But so is the rear fender. The mounting is different to accommodate the larger frame. So, a fender swap at dealer is unlikely.
I know the initial post mentions KSHD, and as said, it is likely a fair amount ended up in industrial applications. But the KS American that was also available would have theoretically been available on sales floors nation wide.
I will say, the HD frame I previously had was pretty well beat. Where as the American I have that came from the original owner is in pretty cherry shape.
I think part of the scarcity is simply people have no idea they have a Schwinn that’s any different from a standard middle weight. (Think outside of enthusiast.) But that isn’t to say they aren’t rare. I was at my local co-op and a guy rode up on a king size American. After trying to buy it off him, and him declining several times, I told him more info about his bike. He had absolutely no idea. And I also know of a few being found for sale with no mention of them being king size.
Although exactly how many where made may never be known. Based on the information we have, dates produced, serial numbers. Estimates can be made, I suppose.