The coin is damaged, it did not leave the mint this way. Damaged after minting, or Post mint damage, Choose your verbiage.
U.K. never used a 12 1/2 per se, but they did use a 1/8 of the base unit - the Half Crown, equal to 2 shillings / 6 pence.
Foxone99 here is one I found I thought I hit the mother load, then I learned how the minting process worked. Dave