I don't know of any "production" knives, but bring custom, I'd bet that Hank/Newt/Eddie @ martinknives.com could do a single guard for you...check out their forum on knife forums. They have one posted there....you might also want to check out Farid Knives on the web. They are costly, but if you are serious about buying, he'd most likely do a single guard.
Cerberus