This is over a short wall from where I had been excavating but appears to be part of the same commanding officer's house circa 180-200 AD. For perspective here is a side view.
Now, the feature with the round hole in the top may not be in its original location. Note the loose stones and rubble underneath it. But the larger stone is the interesting one. It has a slot carved into it.
These turn up in various contexts. Here is a simple water holding tank just inside the west gate of the fort.
And here a much fancier version in the (later, 3rd century) fort headquarters building. It separated an assembly hall from an official room that may among other things been the paymaster's office.
But as to the purpose of a stone screen near the (presumed) corner of the commander's residence, no theories to offer. Other of course the observation that one support stone would not hold a heavy stone screen very well. There should have been a second one but person or persons unknown nicked it a very long time ago.
Moving on from Vindolanda to Rome I was on the lookout for other, similar stones. And of course they kept turning up.
What we have here is a tombstone that at some later date had a slot carved into it. While this may seem highly irreverent it is worth noting that in later eras people did not know how to read Latin. They did however know how to use a chisel. This comes from the very jumbled up site that is the tomb of Cecilia Mattea on the Appian Way.
Now here's one from the Museum at the Baths of Diocletian. Fancy.
And lets finish up with an odd ball.
So my personal opinion that it has an Alien Brain on the top can probably be dismissed out of hand.