Comment by Mr. Medisterpolse:
It looks like Tollefson’s recording was made in London on 31 March 1947, and released that May in Norway with the Lørdagsvalsen title. Two years later, Columbia decided to release it in Britain and the United States as Black Masks. Perhaps with the delay, the Columbia people assumed Svarta Masken was an alternative title and used it instead. Makes sense, Black Masquerade Ball on Saturday? Novel name, increased sales?
Here's SMDB:
https://smdb.kb.se/catalog/search?q=svarta+masken+year%3A1940-1949
In Scotland, the tune Lørdagsvalsen is widely known as Black Mask Waltz. Was Tollefson’s recording wildly popular there, or did someone else have a successful local recording of the song?