Superman-Tim was a series of booklets published from 1942 to 1950 by a company called Tim Publications, Inc. Unlike standard comics of the period, these were smaller, 6″ x 9″ publications with two-color covers and black and white interiors that were meant more as activity books and as promotion for the Tim line of clothing for boys. Later in the series, they featured stories of Superman and his young friend Tim. (Here’s a sample story from Four-Color Shadows.)
This envelope below was used to mail a Superman-Tim club member their copy of Superman-Tim magazine. Because the vast majority of issues of Superman-Tim were obtained in person, this envelope is extremely rare.