A guy I know has an XL600LM, with paperwork to prove it was a Dakar 'client' bike at the time when the XL600R was Honda's works entry. As I understand it, you paid Honda lots of your money, rode it in the Dakar (or maybe other event), then either paid more and kept it or returned it to Honda. The returned bikes were apparently put back into normal trim and sold, which is what happened to Bill's bike.
Can't see from the photo, has it got a starter motor? If it has, it's probably a standard XL600LM (the LM being a 'leccy start version not sold in the US, so if it's a standard bike, it should have one). If it hasn't, then just maybe........
Have you posted in the XL600 thread in Thumpers? Guys there would be all over that bike!
Edit: Hmmmm, just noticed the engine is black, not red. Could have been painted, but I think all the competition bikes had red engines
Pretty soon I knew the tune,
And we sat and sang under the Moon