http://gijoe21.com/printings.htm
First printing :
There are two different versions of the first print: newstand and direct market. While some collectors place a small premium on the price they will pay for a newstand version, both of these versions are first printings.
The direct market version, pictured on the left in the image below, has a white Spider-Man head in the box on the lower left corner of the front cover. When G.I. Joe #21 was first published by Marvel Comics in March 1984, this version was sold exclusively by comic book specialty retaliers, which are commomly referred to as local comic shops.
The newstand version, pictured on the right in the image below, has a UPC code in the box on the lower left corner of the front cover. When G.I. Joe #21 was first published, this version was sold by drugstores, grocery stores, toy stores, convenience stores, and magazine vendors.
There is one other difference between the newstand and direct market versions of the first print. Note that the top left corner of the direct market version has a stylized "M." One pilar of the "M" contains the price and the other pilar contains the issue number and the month the issue was released. The top left corner of the newstand version contains two square boxes as opposed to the stylized "M." One of the boxes contains the 60 cent price, the issue number, and the month the issue was released. The other box contains the comics code approval seal. Also note that the direct market version does not have a comics code approval seal.
Second and Third Printings :
There are several ways to identify a second or third printing. The easiest and most efficient way is the black Spider-Man head in the box on the lower left corner of the front cover, as shown in the image below.
Another way to identify a second or third printing is to check the indicia on the bottom of the first page. Following the publisher and copyright information is a stamp stating the printing number. The image below was taken from a "Third Printing."
Yet another difference between the pritings are the advertisements both on the back cover and inside. For example, both the newstand and direct market versions of the first printing have an ad for the Parker Brothers Frogger video game, as shown on the left side of the image below. The subsequent printings have ads for things other than the Frogger video game. For example, the back cover ad for The Young Astronauts Program on the right side of the image below was taken from a third printing. Interestingly, while the ads on the insde changed with each subsequent printing of the book, the Postbox: The Pit letters page has been consistently reprinted as it appeared in the first printing