Snuck in?

       As it's exactly the same diameter as a 2P piece, I suspect that, even though it long predates decimalisation; it probably snuck in as a 2P coin.
       Curiously any old "Copper" Coins are worth just a tiny weeny bit more as scrap than as a Coin; which, I'm guessing, accounts for their rapid disappearance?
       "There are 6.9 grams of copper in a pre-1992 two-pence piece and 3.45 grams of copper in a pre-1992 penny. At current copper market prices, the copper value of a one-penny coin is £0.0157 pounds, (Which you may recognise better as 1.57P). The copper value of a 2p coin is £0.0314." If you see what I mean.