In the early days, the important thing was to be able to distinguish between zero and oh. There was no standard yet established for the best way to do that. At least it was better than typewriters, which didn't have zero or one keys at all - you had to use the upper-case O and lower-case L instead.

I get that, but this is the opposite of what is normal. This is replacing Os with 0s which I've never seen outside of warez sites in my 40 years with computers.

If you go back far enough, there was no "normal". 40 years might not be far enough back. Different equipment did things differently. Which was O and which was 0 was a matter of convention, there weren't any rules for it and still aren't.

Programmers had been putting slashes through zeros long before The Oregon Trail showed up, and my Mom might even have an old, hand-written IBM coding form lying around to prove it. I imagine in this case it is probably OCR gone awry that confused the zeros and Os.

As one who originally took typing class on manual typewriters, I can tell you that every typewriter I’ve ever used had zeros and ones. If one were to tell me that there existed portable typewriters that did not have those characters, I might believe it.

I never said slashed zeros were unknown, just that conventions hadn't settled yet and were far from universal. Sometime you got a dot in the middle instead of a slash.

I don't know if OCR even existed back when this was published. If it did, it wasn't common.

The manual typewriter I learned to type on did not have a zero or one.