We used these at school. We punched indentations in the sides using a compass point. Obviously that isn't going to work in prison, but you could make marks with

We used these at school. We punched indentations in the sides using a compass point. Obviously that isn't going to work in prison, but you could make marks with a fingernail easily enough. (Of interest, note how the pencil is short in the photo - makes it harder to use as a shank.)