Unknown (also known as Unknown Worlds) was an American pulp magazine, published from 1939 to 1943. It was edited by John W. Campbell who also edited Astounding Science Fiction, which also published some of Fritz’s stories.
Covers are only shown where they are for one of Fritz’s stories.

Two Sought Adventure (aka The Jewels in the Forest)1939 / 08
The Bleak Shore1940 / 11
The Howling Tower1941 / 06
http://www.lankhmar.co.uk/wp-content/uploads/2014/04/UNK_0027.jpgThe Smoke Ghost1941 / 10
The Sunken Land1942 / 02
The Hill and the Hole1942 / 08