[TUHS] etymology of read -u

[Doug McIlroy]

What's the mnmonic significance, if any, of the u in
the bash builtin read -u for reading from a specified
file descriptor? Evidently both f and d had already been
taken in analogy to usage in some other commands.

The best I can think of is u as in "tape unit", which
was common usage back in the days of READ INPUT TAPE 5.
That would make it the work of an old timer, maybe Dave Korn?