> Granted, because I am too lazy to hunt down my 6th Edition
> manual set,

I was not, however, too lazy to remember that I had ftp'd the DEC processor
simulator suite from ftp://ftp.digital.com/pub/DEC/sim/ which includes UNIX
V6 RK05 images.

Even V6 could handle cross-device mv.  I will not bother trying V5.

mv is not the "rename" system call (nor was it the "link" system call
before rename was invented).  mv is a program for moving files.
It should do it, and shut up.