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The old tag is possibly the oldest bollard in London, at St. Helen's Church, Great St. Helens. It's a cannon captured from a French ship in the 18th century, embedded in the ground. I gather this was how the early London bollards were created and you can see how the shape of later bollards echoed it.
At the right time of day, it stands literally in the shadow of the Gherkin. Since this tag is overflowing with phallic imagery, it seems appropriate that the road running from the tag to the Gherkin is called "Undershaft".
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Luckily I have a bizarre memory for random facts, plus the cylinder clue helped.
@Greenbank there's a hidden Batman statue behind some houses in Putney too, but its not accessible to the public I think.
Old:
St Mary Axe.
New:
From old Imperial to new.