As far as I'm aware, we don't see many of them because unmodified they (or is it just the ammo?) are section 5. This one was sect 1 by virtue of it having been modified to .50 BMG cal. I know the bloke who purchased this very well. He was acting on behalf of a customer. He had clearance to bid a lot higher, so someone wanted this badly.
Alan, "good reason" for this piece would be membership of the 50 Cal Shooters Assoc, research, or just collecting. I don't think it'd pass as a foxing rifle though.