> I assume they were referring to the capability at the 1971 date
> where they were used for microcode loading on IBM 370.
And later on many other devices
such as my high school's RJE terminal.
> Only IBM internally could write them at that time.
Similarly, ZIP, Jaz and other magnetic media
could not be low-level formatted outside of the factory.
Kinda like the way servo tracks can only be written
by precision machines, not the home version.