常陸国正宗
It is said to be a sake made by reviving an ancient sake rice called "Watarabune" that had ceased to be produced (see introduction article below).
As the article says, it was quite fruity and delicious. It was my favorite of all the sake we drank that day.
Reviving Sake Rice and Using Wine Technology: Nikkei
https://www.nikkei.com/article/DGXZQOCC042PX0U5A600C2000000/
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