There is a Nov of 71 Chief Counsel intrepretation that is the FAA's final word on the subject. Unfortunately their online stuff only goes back to 1990 and my books only go back to 1975.
From references in later questions, it looks like there have been a couple views in the FAA. It was an issue at a prior job with someone reaching 60 in their grace month. It came down to if you did a look back after his last flight, when was the last PC and ground school? If it was more than 12 months from the last flight, that was a problem.