Dave --  As Ron pointed /dev/ty was part of UNIX early in the game. I'm pretty sure it was in V5 but it might even go back to V3 or V4.

To your point on naming the first serial mux for the PDP-11 was the DH11/DM11, a 16 port [full 'system unit' in the backplate]. The single hex-height DZ board did not appear from DEC until 1977 at the earliest (although my memory is that it was late 78).  The DZ11 is an eight-port mux (with short pinned modem control and no DMA - both causing issues). If you remember, the original 1979 AT&T V7 release tape does not even have a driver for the DZ ( it's in the v7 addendum, which came out 6-9 months later).

I don't know, but maybe tty8 was just simply that the DC11s had originally been to the first eight serial ports, and the first DL/KL was tty8

I'm not sure why I think this ... my hazy memory is that DC11 was 0-7, the DLs 8-15, and DHs started 16

That memory may be because when I set up the the Teklabs 11/70 with 6 Able DH/DMs [which were 16 ports in a hex board each], I used that convention to remember which board controlled which port.