No copyright registration was filed for the issue. As such, there is no official on-sale date. However, May is all but certainly when the issue first went on sale. The official on-sale date for issue #136 was April 29, and it is June 24 for #138. A May on-sale date is consistent with that. Also, at this point in time, comics in Marvel's color newsprint line consistently went on-sale in comics stores four months ahead of the cover date.
There is no Comics Code Authority seal of approval on the cover. According to Marvel scriptwriter (and occasional editor) Roger Stern, the absence of the seal was an accident. "That month, Marvel started running a contest with a big honkin’ banner across the tops of all the covers... which necessitated some special paste-ups... and the code seal either fell off or was never added." Stern was quoted in Comic Book Urban Legends Revealed #93 (March 8, 2007) at www.comicbookresources.com
This issue has variants: