Here's another great place to answer this question http://www.thecomicbookforum.com/forum/comic-book-discussion/comics-general/newsstand-editions-post-yours-here-705

To begin, different printings of monthly comics were first created by Ed Shukin, the Vice President of Marketing at Marvel Comics, in the summer of 1979. Ed was in the unenviable position of having to explain to 500 local magazines distributors scattered around America, all of whom had carved out exclusive regional territories during the distributor wars of the 1930's and 1940's, why Marvel selling directly to comics shops was not going to hurt them.

Comics were already seriously declining in sales at that point in history, so many of the local magazine guys did not really care that much about lost comics sales. But those who did care voiced a complaint that some devious comic shops owners were buying comics (nonreturnable) from Phi Seuling's Seagate Distributing Company at 50% off, and then returning those cheaper copies to their local distributor at a small profit. This was costing them money. To quell these complaints, Ed agreed to remove the bar code from all comics being sold to comics distributors, replacing them with logos, or small ads. Thus the concept of "Direct Market" editions was born.

The beginning of the system of marking comics differently according to distribution system made news in The Comic Reader 167 (April 1979, pp. 5–6). As a matter of historical interest, here is how the news was then reported:

"Those of you who purchase your Marvel comics from comic shops may have noticed that the UPC computer codes on books you got in March have been nullified by a black bar through them. This is evidently because certain dealers who have been purchasing under the pre-payment, non-returnable system from Phil Seuling and Jonni Levas's Seagate Distributors have been returning stripped covers to local distributors for credit they're not entitled to. Also, it appears that the Whitman style editions, with the diamond over the normal number/date box, omitting a cover date entirely, are going to be substituted for all the regular issues for all direct-mail dealers. Since certain of us are picky about which edition we get, there are worries among some of the dealers we've talked to that this measure, done to prevent unethical returns, will ultimately adversely affect their sales."

The earliest Direct Market editions have only a simple slash through the bar code. This began in June of 1979 and possibly lasted three or four issues, depending on the editor in charge, and the publishing frequency of the title. Soon thereafter, Marvel's highly efficient Editor-In-Chief, Jim Shooter, established a unified program of monthly barcode replacement logos, which lasted until he was deposed in a vicious palace coup, in the summer of 1987. After that, as near as anyone can tell, individual editors were allowed to create whatever they felt like in any given month. This decentralized decision-making about the bar code boxes resulted in some titles (such as Alpha Flight (1983)) having bar codes, even though they were really on sale only in the Direct Market. This makes documenting the different printings sometimes very confusing...