[eesmith]

The video was too long for the information content.

Short version: the "@" symbol has been around for hundreds of years. It has different names in different cultures. It wasn't on early typewriters but by the 1889 it was. In the 1960s it became part of ASCII, and Ray Tomlinson used it as a symbol for routing email to another computer.

BTW, they show the "@" on the 4 key. That's the British typewriter layout. The US keyboard had a "$" on that position, and "@" was on the same key as ¢, to the right of ";". (I believe ¢ disappeared in ASCII because it could be composed as 'c' + '/'.)

[enf]

The cent sign didn't make it into ASCII because no one ever made a serious proposal to include it, not through conscious exclusion.

ASCII-1963 did not have Backspace and therefore did not have character composition. The concept of composing accents appeared in ASCII-1965, by which time ¢ was already gone.

[eesmith]

Then I'm mixing up my 1960s technology. PLATO supported character composition, and APL used it for operators like ⍋ (∆, backspace, ∣), so I assumed that composition using backspace was a reasonably widely understood concept which would have affected ASCII.

[enf]

I don't know why they were slow to embrace backspace. Maybe just because most of the control character that did make it in came from Teletype's requirements, but the Model 33 couldn't backspace so they didn't ask for that one.