"Koshu" refers to the north-eastern part of Yamanashi prefecture.
It is also where wine is produced.
On our recent trip to "see" Mt Fuji, we spent a lot of time checking out different omiyage (souvenirs).
One thing we picked up were these scallops that were cooked in wine..."koshu-ni"
For most tsukudani, it is food that is seasoned with shoyu, sugar and mirin. In this case, they use wine in place of mirin and call it "koshu-ni".
You can't taste any alcohol, which is a good thing.
And this is very good with rice. And with beer.
If you visit this part of Japan, you might want to give this a try.