Okay, I figured it out. The numbers stand for which factory it was pressed in. This also answers, in very simple languague, the ongoing question of how to tell if a copy of
Pet Sounds was pressed in Los Angeles or somewhere else (a question which I don't think has ever been fully and exactly answered). According to
http://www.beatlecollectors.com/pages/11/index.htm, the numbers 5, 6, and 7 are Los Angeles pressings. A snowflake type logo on the deadwax also indicates that it is a Los Angeles pressing.