Mattel has made similar buttons for 4 different Hot Wheels releases. The guide below can help you decide which era a given button is from. The four eras are: original redlines, 25th anniversary, 40th anniversary, and 50th anniversary. The picture below shows the Red Baron in three of the four button styles from these eras. On the left side of the picture are the original metal and plastic redline era buttons, on the right bottom, a 25th anniversary plastic, and on the top right a 40th anniversary metal one.
In this era Mattel made both metal (1968-1970) and plastic (1970-1972) buttons. The most important distinguishing feature in this era is the place of manufacture, which is present on all buttons, and is located just after the copyright. It should read either 'Hong Kong' or 'Printed in U.S.A.' for metal buttons, or 'Hong Kong' or 'USA' for plastic buttons.
All of the buttons from this release are plastic and have the same distinctive blue background, as seen above. Redline era plastic buttons have varying colored backgrounds, and none of them match the color of this release. Buttons from this era also lack the copyright printed on the paper insert of the redline era.
For the 40th anniversary, Mattel decided to pay homage to the original metal button. From a distance (or bad Ebay picture) they can look the same, but Mattel wisely put some distinctive visual cues on the newly issued buttons. For one, the font of 'Mattel Hot Wheels' is larger than that of the redline era. Also the copyright notice is different, dated 2007, and is missing the country of manufacture. Another clue are the tread lines on the sidewalls. The redline era had thicker curved ones, while the later issue has straight and finer ones.
For the 50th anniversary, Mattel made it easy to tell these from previously released metal buttons, they printed 50th Anniversary on the front.