Can't say I ever recall hearing that theory. It was a power rating, with A being 1200hp+, so the MetroVick As would have remained As and the 071s would have been As, too, if the system had been retained. It's interesting to note that as the big GMs arrived around the same time as the transition to the new numbering system, they were placed into the same series as the MetroVicks (001-060 and 071-088), rather than being placed into another category altogether (e.g. 901).