I think I've seen some older content where the convention was reversed like this. Wikipedia's article on slashed zero claims that IBM and some other mainframes put their slash on the letter O instead of the number 0. However, in a bit of circularity, its citation (1) is this same BASIC manual. Checking an IBM manual (2) for punching cards, neither 0 nor O has a slash. The manual is undated, but documents the model 26 keypunch from 1949 and not the model 29 keypunch from 1964, so it's probably older than this BASIC manual.

1: https://en.wikipedia.org/wiki/Slashed_zero#cite_ref-9

2: https://archive.org/details/bitsavers_ibmpunchedAnIntroducti... (page 4, 15)