Etymonline states: "The meaning 'pad of writing or blotting paper' is by 1880".[1] Also see the variety of meanings listed it the Webster 1913 edition, including: "1. A small table or flat surface. 2. A flat piece of any material on which to write, paint, draw, or engrave; also, such a piece containing an inscription or a picture. 3. Hence, a small picture; a miniature. [Obs.] 4. pl. A kind of pocket memorandum book. [...]".[2] -- No information on the frequency of use, though.

[1] https://www.etymonline.com/word/tablet

[2] https://www.websters1913.com/words/Tablet