I have to say Max I've used that chart for a decade and it's always been correct for the mid grey series of Mariners from the 70s/80s.
Is your serial no plate like the one below Disschord? If you look there is often a second clue. See the letter stamped in the black area to the left of Made in Japan. That's a date code. I have some notes from my old outboards and I know 1980 was S, 1982 was Q and 1986 was J ... they followed through using a letter a year in reverse alphabet order with some letters omitted.