No copyright registration was filed for the issue. As such, there is no official on-sale date. However, the issue all but certainly first went on sale in March. According to copyright records, the reported on-sale date for issue #134 was February 26, and the date for #136 was April 29. A March on-sale date is consistent with that. Also, at this point in time, comics in Marvel's color-newsprint line were going on sale in comics stores four months ahead of the cover date.
Cover is an homage to the cover of X-Men (Marvel, 1963 series) #56