A private message with Uh, Clem reminds me of another quaint piece of 
UNIX group history:   JHU Ownership.

The original V6 kernel and file systems used a char for UID and GID.    
This meant that you could only have 255 (plus the root user) distinct 
users on the machine.     The JHU PDP-11/45 was used for the EE classes 
and we had more than that many users.    The kernel was modified to 
check if the GID was 200 or greater.   If it was, that was taken along 
with the UID to be part of the user identity.    We gave all the class 
accounts such GIDs.

Of course, we had to be careful about newgrp and fun and games with 
setuid/setgid (both the system call and the bits on the executables).
I spent a lot of my time looking for exploits there and fixing them once 
I (or someone else) found them.