The markings on the stone suggest to me that it might have been part of a pair, possibly for putting bars in? Looks as though they would sort of slot in - can you tell that I know absolutely nothing about this type of thing!? I'm hoping to rectify this with some beginners reading material on the (vast) subject. The only problem with this theory is that logically a person would be much more likely to build fence posts and horse jumps from wood, the work that will have gone into this is far greater than the equivalent in wood, and where is the other half of it?
If you're knowledgeable about this type of thing then please feel free to enlighten me.