UNIX did not have support for virtual file systems (vnodes) until 1986.
- S.R. Kleiman, “Vnodes: An Architecture for Multiple File System Types in Sun UNIX,” Summer USENIX 1986
I remember this quite clearly: Sun needed vnodes to support NFS. After that, the two ways of creating a virtual file system was as a loop-back NFS server (allowing you to write your file system at the user level), and as a kernel-loaded vnode file system (which required rebuilding the kernel because we didn't have loadable kernel modules).
My question: How did Unix before 1986 implement the /dev file system? I don't have a copy of Lions, but perhaps it is time to buy one...