rear sight mechanical zero and index lines

[USGI]

Does anyone else find it hard to adjust the index mark on the Type 3 adjustable sight? I took a wrench from an old Erector set and filed it out to where it would just slip in there. It worked on one of my carbines, but the "jaws" of my wrench spread on me when I tried it on another Type 3 sight that seems a lot tighter. Was there ever a special tool that was made for this? Thanks! - Bob