While I doubt that the lobby card mentioned is "the only one in existence," there are not a lot of them out there. No, while I do have some nice things in my Charlie chan collection, I do not have any lobby cards from "The Black Camel." They do come at a steep price when one appears on eBay!
Rush, Henry, thanks for the reality check. I kinda thought I'd seen them for sale (yes, at huge prices), but not as rare as Mr. Conway states. Thanks for setting the record straight!