I got this from an eBay seller and thought it was helpful:

McCall patterns started putting a copyright date on the envelope in the late 20s. During the 1930s the date is on the flap. Starting in the 40s it is on the bottom edge of the pattern. Just look for the fine print. McCalls is the only one that consistently copyrighted their designs. Simplicity started to put dates on the envelopes in 1939-1941 and then stopped in 1942, and started again in 1943 but the copyright date was moved to the instructions. Other patterns have patent dates but these can only pinpoint the earliest date the pattern could be based on the most recent patent date. Vogue started putting dates on in 1950 on the bottom front of the envelope eventually they moved them to the back sometime in the late 50s. Butterick never had dates but I have heard that if you email them with a specific pattern number they will tell you when it was first printed.

-- Chrissy Hall Reis (hallreis@yahoo.com), October 31, 2001