(I reckon you could be right! Looks like a good find. It could be natural, but I'd have to take a good look at it. It could be a broken basin stone.>

Didn't want to say it in the iamge info. but it struck me as very similar to the basin stone in B1. It's a good 200 yards away from the fort. I don't think it's natural but I would defer to your better knowledge..