The type 55 Bing has a Magnesium body, and flow even better than the 54. I have seen only 38 40 and 44mm sizes. I used a type 55 44mm Bing on my Honda CR500 ( the Bing was also a hop up for CR's) to many hillclimb wins, including the 87 AMA amatuer top time, and the Muskegon Mi AMA Pro hillclimb in 1988.
Contrary to all the articles from way back about Bing's, the Bing's are way easier to jet right, easier starting, more consistent running, and more power can be had than the Mikuni replacements. I have a flat slide Mikuni on my 320 Maico, and it is a bear to start, and it runs different (too lean to too rich) from morning to afternnon, so the Bing is going back on.
The only issue with a Magnesium carb is they wear out faster than an aluminum carb in the slide area, also, don't try to use ethanol or methanol fuels, it will destroy the magnesium.
Paul