The Viking Symbol Mystery is Volume 42 in the original The Hardy Boys Mystery Stories published by Grosset & Dunlap.
This book was written for the Stratemeyer Syndicate by Alistair M. Hunter in 1963.
The Hardy Boys and Chet Morton travel to Canada's Northwest Territories to recover a stolen Viking artifact (a runestone). They also smash a group of thieves robbing recreational lodges around the Great Slave Lake. They visit Saskatoon, Saskatchewan; Edmonton, Alberta; Fort Smith, Northwest Territories; and Hay River, Northwest Territories.