A letterpress make-ready sheet. These sheets are just successive single proofs, printed in a various single colours through many passes on the press until they become mostly ink and consequently unfit for make-ready anymore.
What you are left with after this process is an abstract work of typographic art that is completely random but quite fabulous. Strangely they begin to look like a stained glass window, particularly when strong, vibrant colours are used.
Quite a few people have tried to buy these prints, but I just can't bear to part with them.