The original small block "657" grind from Racer Brown was 312/606". They were 271 @ .050. The newer non Racer Brown MP is 312/ 590. I have both here. I also have a new MP 590 from last fall for a big block that measures 271 @ .050. Did you measure your new one? Part of the confusion comes from the old literature stating that the @ .050 duration is 85% of advertised,but MP used advertised duration numbers from .020 instead of .015 like everybody else.
Keith