Walter E. Floyd (1861-1940) was a popular stage mentalist who performed a strong two-person mindreading act with his wife, Mary. The Floyds enjoyed a long career, and by 1921 were considered one of the top acts in their field. Like Houdini, Walter was very interested in exposing "fake mediums". The two magicians sometimes worked together, sharing information and ideas.
This envelope, dated 1921, bears Houdini's handwritten note in pencil that says "Anna Eva Fay". Houdini worked closely with Anna Eva Fay at this time, as she was able to educate Houdini on the many methods used by the spirit mediums. The envelope is also marked "Important" in red ink. Apparently the contents was something to be shared with Anna Eva Fay.
The envelope also bears the official seal of the Society of American Magicians. W.E. Floyd was an active member, and Houdini was the president of the group.