Geograph - photograph every grid square

Beside the path along the northern edge of Croham Hurst is this unusual metal post.
I have no idea what it's for unless it's a parish boundary mark or something like that.
Any info welcome, and I'll edit the caption.
Updates:- 'I would think you are correct in your assumption that the bollard is a boundary post. The old series 25k OS, available as a pull down map on the left side of the Where's the Path website, shows a "BP" here on the civil parish boundary between Croydon to the west and Sanderstead to the east." Thanks, Nigel Cox.
And "Looking at Old Maps Link there are several marked in the area on 1:2,500 maps from 1897 onwards, running north and west from this point. They mark the County and Parliamentary Borough Boundaries." Thanks, Penny Mayes.
And "There are boundary markers in Croham Hurst, but not at the grid reference for this post. This 1:25" map shows the boundary line with some BP's marked. I think it's the boundary between Croydon County Borough and Addington parish. There's a boundary marker at TQ3380963507 Link (Archive Link ) ." Thanks, Mike Rayner