The book has a misprint. My God, what difference does it make? You're worrying about a .001" on an inch difference, do you realize that? ONE THOUSANDTH of an inch. You've already been told by more than one reputable person that the .509 was the big block grind and the .508 was the small block. It's a great cam but it's REALLY big for a hydraulic. You need at least 10.5:1 compression for it to run properly and give the power it was designed to give. It has 248.5* of duration @ .050" lift ground on a 108 LSA so don't expect a vacuum signal worth snot and expect a VERY rough idle. It will need a 3000-3500 converter and at the VERY least 3.91 gears and prefferably 4.10 or 4.30. It is a race bred hydraulic cam and runs that way, but it's a damn goodun.