The reason you cannot find much about this coin is because they don't actually exist, unfortunately. There is no "50 Mark" gold coin. I believe you'll find the portrait of Kaiser Wilhelm on the obverse, who didn't ascend the throne until 1888. This is a known "fantasy" piece.
I fully agree with the others. It's a medal, not a coin and certainly not made of gold. And it's not "Kong" like in King Kong but "König" which means king in German.
Thanks I have had it for 40 years in a jewelry box, and I didn't think it was gold. It belonged to my grandmother born 1912 in Germany. I will pass it on to my daughter who collects coins.
As SuperDave wrote, this is not a coin but a fantasy piece. Apart from the "wrong emperor", in 1878 the large or wide eagle was not used on actual coins. That was introduced in 1890. Such pieces were usually made as buttons or pendants, or for bracelets, etc. Also note that the mint mark was left out by whoever had this made. I would keep it, or give it to your daughter as you mentioned - not because of any metal value (I don't believe it is gold either) but because it is something that reminds you of your grandmother. Christian
Speaking of gold coins, does any body know of a good site where one can get current prices for Word Gold Coins. I have been using NGC for some reference, although they don't list that many and stop at S on the list. Thanks