I saw this on Craigslist and was going to pull the trigger until I saw smoke stack on the front? Did I miss something? Did Lang's ever have a front stack? I'm 99.999999% sure the guy is a fraud as he said he went down to ALABAMA to pick it up from the factory.
From the looks of the welds I'd say somebody added it, though I'd be curious as to why. Seems like it'd pretty much eradicate the whole "reverse flow" idea. Otherwise it looks real, and there is a phone number in the ad. That price looks a little too good to be true though. And it's kinda hard to mistake SE GA for Alabama.
As Mdboatbum indicated it's probably something the owner added for no apparent reason. If the price is right and you can get him to go a litter lower for the mess he made, you could easily cut it off and cap it or for that matter just never open the added chimney.