Mark's correct, it's just part of the old style repeat sign. It encloses the part to be repeated. In a piece or two later you can see it done slightly different as both sections are repeated.
The slashes are considered redundant these days although I've occassionally seen composers use them as they hand write their manuscript.
Scot Tremblay Guitars
"One picture is worth a thousand words. So, for me, one good note put where it should be put, will say what it will take some people many notes to say. ~B.B. King, 1986