Actually I know what it is, and how it was used. But what it's called has fled my addled old noggin. I'm not a good internet searcher, so maybe some old newspaper hand here can remind me of the proper term for it.
Are these what Holmes referred to as the "Agony Columns"?
Looks like what newspapers used to cast the printing plates with. I don't know what material they used to cast the plates with, obviously it was not molten. I delivered a weekly paper in the late 60's and the newspaper bundles came wrapped in these in wet weather
From museum of Printing: To have multiple presses running hand comp would require multiple settings (and multiple errors and proofings). The solution to this problem is the stereotype (or in French, the cliché), by which each multi-page form, in a special chase, is cast into a solid thin metal plate, which may be printed either upon patent bases on standard presses or on rotary presses built for stereo plates. Multiple plates can thus be made from one setting (hence stereo); and once the plates are made, the type can be distributed or melted for reuse. Mats taken from the type form can be expressed to remote sites for casting, allowing coordinated advertising and syndicated copy without resetting.
Is that just an old metal printing plate?
I think what it was called is a Stereotype plate. Flat type from the linotype was pressed into the damp mat then wrapped around a drum, where lead was poured over the mat producing a curved printing plate for the rotary press.
I think what you have is called the Matrix (paper mache mat). That is the positive into which the lead is cast to produce the stereotype plate, after being wrapped around the casting drum.
Is it made out of hard paper/cardboard?
I think they used to be called a "Flong".
In Oz anyway.
Manuel in Oz
Steve, having been in the newspaper business for almost 30 years, when I saw the photo, it brought back a lot of memories. The old printers called it a "mat". This was used in a semi-circular mold in which a molten (yes molten) mixture of lead/tin/zinc (almost like Babbitt) was poured to form the printing plate that was attached to huge cylinders on a web-fed newspaper press. It was a dense and highly fibrous cardboard type material that would handle the intense heat of molten casting. When the material was fresh, you would have a hard time cutting it with scissors or knife.
I can report that after eighty-three years it cuts easily with scissors.
When my grandfather was building the house he had to use whatever he could get cheap or free. Instead of buying 30# felt to lay under roofing or for lining walls, he got these free at the Traveler and tacked them on. Working on the house, in most places I'm using the felt because it doesn't show. But in rebuilding the back porch I'm lining the inside of this wall the same way he did in 1932.