Not to put too fine a point on it, Kirk's filesystem does not show up in the mainstream outside of Berkeley until 4.2BSD IIRC mid 1984. 4.1 was still based on the V7/TSS file systems [inside UCB we had 4.1A, 4.1B, 4.1C - although if my memory serves me 4.1C was semi-available - I know I took it Masscomp, SUN had it, and Armando you must have had it at DEC].
Anyway, the post 4.1 BSD system was when Fast File systems and Berkeley directory system calls were added, to make some of the operations atomic and the user space code more portable.
Henry Spencer's famous quip in the early 1980s: "4.2BSD does everything UNIX does, only differently."
Looking back on it, ideas like the VFS layer would take a few years more. But without moving the directory specifics out of user space code like it was V7 and earlier, it would have been hard to create something as clean as VFS.