I've found info that the Phantom has appeared in Seura at least since 1959.
I believe the Phantom arrived when Ratsupoliisi King (King of the Royal Mounted), which was in Seura already in the 1930s, ran out of new material after the middle 1950s. However, King came pretty soon back to Seura as readers demanded it, and since that he has been published as reprints in the magazine.
Maybe Lurker knows the starting year of the Phantom in Seura? He knows about Seura’s old comic strips and has a blog about old popular culture. His Populaari blog says that already Seura 45/1950 had a
cover where the Phantom appeared! The cover text says: “A thrilling moment”. The Phantom was known in some Finnish newspapers already in the 1940s.
Seura has always been a big magazine. Circulation is still about 150 000 copies.
I have collected some Seura’s strips. Here is an old Sunday strip around 1966. It is the beginning of the story “Wamba Falls Inn”. Colours are modest.