>How could things have flipped 180 degrees like that?

Somebody other than IBM came along?

IIRC electronic communication started with just ones & zeros then later got the rest of the numerals.

Eventually they got (upper case only) alphabetic characters and by that time a plain zero had already been well established.

Teletypes made sure the letter O always appeared different than the numeral zero.

I never actually used Teletypes until the 1980s where we still got official cables by Telex. But even then a large number of experienced administrators had learned to type manually on traditional typewriter keyboards which are also hard to press but there was no zero or 1 key when they learned. It was important not to reflexively substitute letters for numbers, and if you did you needed to be able to tell the difference on paper.

The ones they are talking about are some of the earliest ASCII Teletypes, where the numerals were wisely standardized equal to the least significant 4 bits out of their total of 7 bits. Zero is 0000, and upper case O coincidentally is 1111.

IBM had its own standard before ASCII, apparently IBM printers produced a slashed upper case O before that.

At Dartmouth it looks like the Teletype Model 35's they had were combining or overprinting the regular slash and zero fonts to make upper case O IBM style, and it appears their keyboards reflected the slashed output. The actual printouts show this.

The main part of the documentation looks like the typical output of a manual electric IBM typewriter instead, where they slashed the O by backspacing. Looks even more funky compared to the real printout. Any typos would have been corrected the modern way, painting over them with Liquid Paper then retyping after it dried.

The original printouts didn't always need to be actually cut with scissors & pasted into the document like lots of ordinary graphics since sometimes it was a whole page, they just manually typed a page number at the bottom and it got Xeroxed into its place. Others were probably cut with a paper cutter beforehand, not scissors.

ASCII had early revisions about this time, within a few years I was a youngster in an office where we got one of the first "luggable" printing terminals and connected to a mainframe through a telephone modem over long-distance AT&T lines. With the suitcase model and acoustic coupler modems we could log in to the "host" with our "laptop" from clients' offices or residences to serve them with more computing power than anyone else around.

These were some of the first that were made for computer use not for the Teletype network. Just an early RS-232 connector with 6-conductor cable to the DB-25 socket on the modem. RS-232B at first then RS-232C, still waiting for RS-232D, expected after they landed on the moon. They weren't quick enough and IBM put RS-232C on the desktop, called it a "COM" port and its been that way ever since. Well if you still want to have the remaining possibility of doing fully-backward-compatible serial communication, and without needing hardware-specific drivers to communicate with any vintage of computer or modem.

This was the first time I saw the slashed zero, never the slashed upper case O. And it was a traditional non-typewheel mechanism, electically powered so the keyboard was not mechanical, but otherwise it made as much noise as a really fast hard-hitting typewriter, making multiple copies on modern carbonless triplicate rolls.

People were amazed with new technology back then.

Eventually with PC's a number of office people connected external modems to their standard COM ports to log into servers, usually plugged right into a modular Bell system phone jack rather than acoustically coupled to the standard telephone handset, plus autodialing became available. Their standard serial mouse was on their other COM port. If they had DOS software that supported a mouse, which was not yet common.

As a web began to appear these were the only people already equipped to partake, but in the long run the net got popular and built-in modems were common expansion cards (later standard equipment way before RJ45 ethernet) which connect right to the phone jack without needing a physical COM port. Plus the next modems were built into the motherboards which all just had phone jacks for a while, mostly software modems.