Rolf,
The original B-Class rules (circa 1960) were very basic -- maximum 20 feet in length, maximum 10 feet in width, maximum of 235 square feet of sail area. I can't remember if there was a minimum weight.

I'm quite certain that the Tornado was originally designed (about 1967) to exactly fit the B-Class. The Tornado is now, of course, an international class on its own, and changes the class has made have nothing to do with the old B-Class, which no longer exists as far as ISAF is concerned.