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In the manual they have the symbols for zero (0) and the letter O switched.

This leads to the statement : F0R X=1 T0 1OO.

Was it really that way back(wards) in 1964?

It's most likely legacy from pre-computer unit record equipment. These machines could only handle numbers and printed zeros without a slash because there was nothing to confuse them with. When letters were later added, it was the new character that got the slash.

Additional citation hunting from 2020 when the BASIC manual was shared & discussed here:

https://news.ycombinator.com/item?id=25462835

Reply on news.ycombinator.com