The select system call was added in 81/02/07 with no comment. Commit
history shows in 81/10/17: "cleanup (mpx removal, old tty removal,
beginnings of select)" and in 81/10/11 "first boot with select()" which
includes lots of changes like replace lots of tty code and use
selwakeup().
Actually, that is a nice memory jog. Chesson wrote mpx(2) for DataKit for UNIX/TS. IIRC: It was not originally part of v7, but he sent copies of out to a number of folks. Somwhere I might even have the email when he sent it to me at CMU in the late 1970s. The point is that it was in the wild as it were at a lot of places; certainly at UCB by 4.1. Sam and Joy had seen and messed with mpx(2) before select(2) was concieved.
To Paul's question - mpx(2) was done for networking.