As Bike Bum said, you can't patent a design as such. If what I heard and remember is correct, there used to be such a thing, but they turned out to be unenforceable and they stopped allowing them decades ago. The patents were based strictly on the drawings presented to the patent office as I would imagine allowing a broad design patent would quickly bring any innovation to a stall, but that meant that all it would take for an imitator to get around the patent would be to alter a few lines in a manner that could be imperceptible to the end user, but still diverge from the patented design enough to not fall under violation. I would figure, it also held the patent holder to the exact drawing and that any alterations during production or variances in manufacturing could arguably have to be patented separately to be protected. Also, policing a patent can be quite expensive and it is necessary to retain ownership. The design of the tank can be trademarked, but would also require trademark holder to police it, which if someone decided to steal the design, would probably cost more to fight than would be worth it. That said, as cool as these tanks are, the market for them is small (and the thieves would probably be quickly rooted out by people on here!), so I doubt policing it would be necessary.
