In 1844, French author Alexandre Dumas published the first of the d'Artagnan Romances - the Three Musketeers. He wrote in collaboration with his longterm ghostwriter, Auguste Marquet (who really should be on the cover with him at this point).

The book and its sequels, Twenty Years After and The Vicomte of Bragelonne: Ten Years Later. (If you're asking where The Man in the Iron Mask is, The Vicomte of Bragelonne was so long it was split into volumes, and The Man in the Iron Mask is the third or fourth of them) are historical novels...Dumas lived in the 19th century and the books were set in the 17th.

If somebody published something like this, it would be put on the historical fiction shelves. So why am I arguing that Dumas' work falls under the SF/F umbrella.

It's because we've all read it.