1967 Barracuda Formula S 383

Could be they just bumped the number. The big three all fudged the HP numbers DOWN for insurance (and to a degree NHRA) purposes, so why not just move one back UP if it had been fudged to begin with.

And yes, the 906 has a bigger chamber than a 915 or 516. But it really isn't that much bigger. Those closed heads had bigger chambers (as delivered) than people think.