"Thaler" was a denomination used in the Austrian, German, and French States, the Swiss Cantons, and a dozen other countries during the 18th Century - a total of 100+/- separate entities. "A" was a mint mark for Paris and Vienna, among others, during parts of that century; but each potential issuing entity needs to be checked for possible use of that mark. "Eine" is German for 1, so perhaps we can eliminate the French States and a few of the other countries from consideration. Unfortunately, the remaining possibilities take up about two-thirds of the 1,236-page 3rd Ed. 18th Century SCWC. The "24" may indicate that it's a fractional Thaler, which would further reduce the candidates, but Chip, I'm afraid that unless someone specifically recognizes your coin, or at least its country of origin, it would take a huge investment of time to compare your pictures with all the possibilities - much more time than I have available. (The crowned "R" has a Scandanavia feel to it, but Krause does not list any Scandanavian issuers of thalers. )
It is a silver 1/24 Thaler coin from Prussia minted in 1782. Minted under Friedrich II. The obverse of the coin has a crowned "FR" monogram (for Friedrich II) with the date to each side of the monogram and the reverse has the denomination above an open wreath. Minted in Berlin "A" mintmark on the reverse. KM#296 in Krause
