What, exactly, do the OS mean when they refer to one of their pillar's smaller brethren as a 'rivet', 'bolt' or 'surface block'?
There seems to be very little consistency regarding how their various bits of ironmongery are labelled and in fact, the terms seem to be pretty much interchangeable.
I've found surface blocks that are bronze hemi-spheres, a couple of inches in diameter set into a lump of concrete at the side of a country road. I've also found surface blocks that are tiny little rivety things about 1/4" in diameter set into the concrete surround of a drain in a public car park. There are also examples of both of these types being called 'rivets' and occasionally 'bolts'
I'm not particularly bothered as they all count but I am just a little curious about it.