Could someone clarify 12 vs 18 month period?

Specifically, The Motley Fool Investment Guide (1966) said the distinguishing period is 12 months.

Then, The Motley Fool's You Have More Than You Think (1968) changed the Foolish Four Approach to recommend holding for 18 months because of the 1997 tax laws (p. 194 in the soft cover book).

But now, The Foolish Four book (1998), as well as excerpts from the Investment Guide on this site all say 12 months?

Did the law change from 12 to 18 months, and then change back?

Thanks,

John Fields