(Did you mean a Master of Ditto?)
A ditto or spirit duplicator master, equivalent to a mimeo stencil, is a three-layer form consisting of a shiny sheet of heavy paper, a "carbon" sheet with an aniline dye coating and a tissue sheet separating the two. The protective tissue is removed, roll the other two sheets into your typer, type on the paper and letters copy from the carbon on the paper's back side, which is then wrapped around the cylinder of your ditto machine for printing.
If you make a typo, you scrape it off with a razor blade, place a fresh piece of carbon over the spot and retype it.
The same masters can be used for hectography.
The "carbon" is commonly purple (the color of Ghu), which is the most vibrant. Other colors, including red, black-ish, green, blue and the rare yellow and brown, were once available but are no longer made.
A special kind of heat-sensitive master is used with a Thermograph, a machine that uses heat to transfer copy and art on plain paper to the master, which can then be used like an ordinary master.
Also involved: - Cut - Ditto Masters - Ditto masters - Ditto-master - Ditto-masters - Hekto - Lettering Guide - Shading Plate - Stylus