The most important part of the legend is that the bell works OK if you buy one for yourself, but its powers are doubled if it's given to you as a gift!
It's also not Harley specific. I saw them on a lot of used bikes when I was looking awhile back.
This raises the question - should you take off the bell if you sell your bike or does it stay on THAT bike forever, continuing to ward off the evil spirits? Are the powers diminished if you move it from one bike to another?