It's definitely an original Yoshi, I've even got an original catalogue from the late 70s somewhere with it in (they claimed quite a few bhp gain at the the top end too). And a spare brand new number 3 pipe, which might come in useful sometime. I have only ever seen them in chrome finish- I guess yours has been stripped?
Oddly enough, the style of Yoshimura 400F exhausts that the likes of MC-Again sell nowadays seems to be a modern invention, I've never seen a 1970s/80s Yoshimura 400f exhaust like those. They are actually much more like the Yoshima exhaust, which Marving have rather blatently copied too.