This horn does exist. I have seen and played one in Japan a few years ago. It was in the Yamaha Atellier shop in Ginza, Tokyo. It was gold brass and 4 valves. It played pretty well but there are many great BBb tubas so I did not give it much attention at the time. It was certainly above average and definitely the best BBb Yamaha I had played. Anyway, they are custom orders. Don't know if they would make one with 5th valve but would be a good idea!! Hope that helps,
Chris Olka
Seattle Symphony
Seattle Opera