Please excuse that I can only answer in English. Here is the listing for your coin
N#157890
Most of the time when people ask about an old coin, the answer is disappointing, maybe 1-10 dollars, maybe nothing, and lately, with the price of silver, just the melt value. But your coin might be an exception. The page does not list prices, but that is usual for older, less common German coins. The Numista Rarity index listed on the page is 100, so this is the most rare coin by that measurement.
I went looking for sale prices. https://www.numisbids.com/sale/8905/lot/645 This one sold for 2500 euro. The condition of the coin is very important to the price. I am not an expert on the grading of coins, so I can't tell you what your coin is worth. DO NOT TRY TO CLEAN YOUR COIN. That will reduce the value, unless recommended by an expert, and done professionally.
Nothing about your coin looks like a counterfeit to me, but you always have to be suspicious with a valuable coin.
Since you are in Germany, you could look for an established coin dealer or contact a coin auction company. If I was you I would also ask for advice from the German coin experts at https://www.numismatikforum.de/ They will have better knowledge of where you could get a reliable estimate of the coins value. They are also very good at pulling up auction results.
Congratulations