There's a famous US counterfeiting case that was broken during the War. A bank teller who also was a coin collector noticed a "war nickel" without the mint mark. Eventually the Secret Service investigated and discovered a counterfeiter who was making nickels! He figured nobody would ever notice they were fake because they were just nickels.
The dies were excellent and the machines he used so professional that after his conviction his equipment was installed in the US Mint.
Had he remembered the mint mark perhaps he never would have been caught.
Good luck to all,
The Old Bookaroo, CM