Here is a mail order dress pattern, The Household Pattern, mailed from Brooklyn, N.Y. in a plain brown envelope mailer.  The postmark is stamped 1917 so certainly dates this dress pattern around the era of World War 1.  What a marvelous style  ~  slip over the head, in a one-piece with gathered skirt.  The waistline is raised forming a drapery at sides.  Long sleeve with long pointed collar. The pattern is made to fit ages 16 years, 18 years and 20 years of age.  Bust 33, 35, and 36".  Hip 36, 37 and 38.  Each pattern piece is marked with a letter and all pieces accounted for.  On the printed layout, each piece is marked with a name and corresponding letter but on the pattern pieces itself it is just the letter.  Directions for sewing are on the layout sheet.  

Condition is very decent.  Unmarked tissue paper pieces with letter markings.  Plain mailing envelope is certainly not perfect,but for the age good.  The paper showing the dress pattern with instructions is a single page with printing on both sides.