It's really fortunate that the history of the 555 timer is really well documented. Its inventor, Hans Camenzind, wrote several books, and even had a Youtube channel in his later years[1]. It's a shame that so many iconic chips that have changed the world aren't so well documented. I went down a real rabbithole a while ago trying to find in-depth information about the Hitachi HD44780. I couldn't even decisively pin down exactly what year it was first manufactured. It's interesting to think of microchip designs as a kind of artistic legacy: Chips like the 555 have had an enormous impact on modern history.
That 1981 document says "preliminary", which suggests very limited trial production. Even the 1985 reference I found is "advance". First tape-out may have been earlier than that. Have you tried asking Hitachi (apparently now Renesas) about it? It's more likely that this information is available on the Japanese part of the internet.
