"iron rolled in large, flat plates for use in making steam boilers," 1840, from boiler + plate (n.). In newspaper (and now information technology) slang, "unit of writing that can be used over and over without change," 1893. The connecting notion probably is sturdiness or reusability, but it might also be literal: From 1890s to 1950s, publicity items were cast or stamped in metal ready for the printing press and distributed to country newspapers as filler. The largest supplier was Western Newspaper Union.
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