I followed a chain of linked articles from OP and eventually found an explanation on an article for Ozyorsk[1]: > Until 1994, it was known as Chelyabinsk-65, a

I followed a chain of linked articles from OP and eventually found an explanation on an article for Ozyorsk[1]: > Until 1994, it was known as Chelyabinsk-65, and even earlier, as Chelyabinsk-40 (the digits are the last digits of the postal code, and the name is that of the nearest big city; which was a common practice of giving names to closed towns). [1]: https://en.wikipedia.org/wiki/Ozyorsk,_Chelyabinsk_Oblast https://en.wikipedia.org/wiki/Ozyorsk,_Chelyabinsk_Oblast