I always though it had to do with the size of the block casting it self.
Back in the day GM had a 400 Cube small block and a 396 big block.
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-Jim
“Equality, rightly understood as our founding fathers understood it, leads to liberty and to the emancipation of creative differences; wrongly understood, as it has been so tragically in our time, it leads first to conformity and then to despotism”-Barry Goldwater