Rex Stout Mystery Quarterly

Rex Stout Mystery Quarterly is a digest sized magazine published in the U.S. which was launched in May 1945. Each issue featured an introduction by Rex Stout and a novella sized story plus other short stories, usually reprints. The magazine underwent a few name changes, becoming Rex Stout Mystery Magazine and finally Rex Stout's Mystery Monthly.

Only the first incarnation is relevant to Agatha Christie. The magazine only carried one of her stories, in its first issue:
 * no. 1, May 1945 - The Four Suspects