The stereotype system was developed by printers around 1800 to avoid the problems of type wearing out on the press during a long run, thus needing to be set again at great expense.
The page of type is moulded into a soft material that will harden, then called a matrix. Plaster of Paris was used in the early days. When hardened, molten typemetal is poured into it producing a plate that can be printed from in place of the typesetting. Many stereotypes could be made from the one original setting, enabling unlimited printing without resetting.
In the 20th C, stereo mats were commonly made from a multi-layer paper board. A large number of plates could be cast from the one matrix. Stereotyped advertisements were the backbone of the advertising industry for some decades.
MMOP has thousands of stereo mats and thousands of stereo blocks. Here is a mat with one of the eight blocks made from it. They are all visible in the MMOP showroom.