Typo. I have the book and there a couple of glaring editorial errors in the book in general.
What is the barrel date code on your rifle? LHS just ahead of the receiver? There will be 4 characters/symbols. It's the two middle ones (letters) that we need to know. If more than 4, the 3rd and 2nd last characters (letters). Just to clarify, I count the last character as ONE symbol, even though it MAY be a 2 digit number such as 30 or 63.
If it's marked SS it is built in '63 or later.