We had these in the late 60's when doing maintenance flight tests on the Lightning. The test schedule came as a long paper sheet which we wound onto a knee pad. I am not sure if these knee pads were RAF issue or whether they were made in the workshops. Perhaps you should also try on the Military thread as someone may have one stashed away. Otherwise I don't think it would be too hard to make one - 2 rollers on a metal sheet with a strap to attach it to your knee.