Casio CS-82 / CS-821 Manual — Module 231

One owner's manual PDF for the Casio module 231 in English. Casio fitted it in 4 models, including the CS-82 and CS-821 — the full list is below. Casio's spec sheet lists one 391, with 15 months of battery life and accuracy within ±15 seconds a month; the module first shipped in Aug 1982.