Mawz: indeed. The grove is missing. I thought I was connecting the adaptor the wrong way, and spend half a day today playing around with it.
Since I have nothing to lose, I will file around, but the chances are that my filing will be uneven. The Adaptall-Nikkor adaptor locks perfectly - it got 2 groves into which locks on the lens are gripping and locking.
I will see if I find any other unchipped adaptall adaptors on ebay today...