It might be, but if so I don't think it's a 77/78 as the seller states, I think I'm correct in saying that Siennaburst was very much an 80's thing. And certainly every Siennaburst I've seen has a standard board (i.e., no block inlays). We should never discount the idea that Fender found an unused 70's neck and stuck it on a new 80's instrument of course, but the bass is so far from original anyway that I doubt it the question of whether that body left the factory with that neck could be established to any reasonable degree. Given this I suspect that in marketplace terms it's a bitsa, and as you say, around £2000 too expensive 👍