The bollards (above/below) have been planted with one high and one slightly lower which gives the impression of keys on a trumpet (possibly). Each of the bollards has a square shaped base with the main body of the bollard having a circular base that then rises vertically with a ridged body which is finally topped off with what looks like a 'curling' stone but without the handle. The bollard top is painted silver with the main body black.
Take a close look at the base of the bollards (below) and you see that every other one is planted differently.
Continuing our #guestbollard feature I once again must thank @nancyo on the twitter for these beautiful bike bollards (below) which can be found in Ipanema, Brazil and just in case you've started to hum that beautiful song/tune here's the link for 'The Girl from Ipanema'.
Bollards of London (and Ipanema)...