This also came in the year end post today. It's a 1626 HI (Not sure what the HI stands for???) Silver Groschen. Happy New Year Bone
In this case the letters "HI" represent Hans Jacob, who was the Mintmaster at the Chemnitz Mint from 1624-35, during the rule of the first Johann Georg (there were four). That was during the time that the Roman alphabet had only 24 letters, lacking "J" and "U". In 1666-67, during the rule of Johann Georg II, Mintmaster Hennig Idlers used the same HI mark at the Bautzen Mint. The study of 17th-19th Century German State coins involves a huge amount of information.