Hi Numisma & Coinfrog & JimmyD,
Thanks for your help and quick replies.
I also believe it is post-mint damage during circulation as such error (if it was) should have been easily picked up (and disposed as waste) by
The Royal Mint, even not that should be discovered at the very early circulation stage.
My query is that the surface around the three indented sides are flat. When the metal being displaced by hit / impact, I thought the surface will pop up at least a little bit.