I know this coin is a Polish, Sigismund III,AD. 1587 to 1632 3 Groschen...What I need to know is the date? I notice on the reverse at the bottom looks like 91.I know that on some of these coins they only have the last 2 numbers on a date. I was wondering if the date of this particular coin is 1591? Any help will be appreciated!

Thanks!
Frank
