In early printing, a plate would be assembled from individual letters of type before it was used for printing. However, it quickly became apparent that certain words or phrases were used a lot, so typesetters would create "a solid plate of type" for those words or phrases to speed up the typesetting process.
before the stereotype process, you printed from the individual letters of type, as people still do today for flyers
https://en.wikipedia.org/wiki/Stereotype_%28printing%29 https://en.wikipedia.org/wiki/Letterpress_printing
A large part of the printing business was to print fill-in-the-blank business stationary, like orders, invoices, etc.
So a stereotype came to mean a standard form that you could adapt to whatever context by just changing a few details.
1. https://www.smithsonianmag.com/innovation/sterographs-origin...
it’s correctly attributed, just one hypertext layer deeper
https://www.etymonline.com/word/stereotype
1798, "method of printing from a plate," from French stéréotype (adj.) "printed by means of a solid plate of type," from Greek stereos "solid" (see stereo-) + French type "type" (see type (n.)). Meaning "a stereotype plate" is from 1817. Meaning "image perpetuated without change" is first recorded 1850, from the verb in this sense.
https://www.etymonline.com/word/cliche
1825, "electrotype, stereotype," from French cliché, a technical word in printer's jargon for "stereotype block," noun use of past participle of clicher "to click" (18c.), supposedly echoic of the sound of a mold striking metal (compare native click).