From barefoot of the XLForum:
Harley never made any XR frames. They were ALL made by “some outside company.” Nichols Engineering did a bunch, maybe some other places as well. Terry (Knight) may have made the late “factory” frames, I don't know. The very first 'XR' frame was built by Jim Belland, by modifying a Sportster frame. At that time in Class C, aftermarket frames were not allowed. The history of the bike is much more important than trying to say “this is a factory XR.” If it was run by anyone with any chance at all, it's not stock. My dirt frame was a Lawwill discard, it had an inch added to the down tubes and an inch to the top tube. They were all modified … the sharp guys would walk around the pits eyeing each others' stuff, playing head games “Oh, I see you dropped the swingarm pivot a quarter inch. How's that working out”? So unless it was never raced, it's probably not stock. ¹⁾