It must have been superstones. The Naniwa Traditional series only has the 220.
And I've heard many times that the Superstone 400 should be quite good.
I've just looked at the picture I took of it, and you're absolutely right - it says 'S400' on the side, which I think is the Superstone.
Good to know that my impression chimes with that of others - I thought the 400 was very good indeed. The 1k side of it less so, but still miles better than my 3k.