Thanks to both of you for the responses. The book is "The Complete Idiot's Guide to Playing Rock Guitar" by David Hodge, and the particular exercise is a little song called "Open Chord Rock." I'll take a picture of it when I get back home this evening; I'll also check to see if it's perhaps just a forward slash (my memory is that it truly looks like a 7 superimposed on top of a 3, but it will be interesting to check).

Thanks again!