This issue has variants.
The on-sale date is the publication date reported in the U. S. Copyright Office filing.
An earlier indexer provided an on-sale date of May 10. No citation was given, although other indexers speculate that it may come from a listing in the fanzine The Comics Reader.
Speculation has it that In June 1977, Marvel Comics wanted to test the market to see if comic buyers would accept a five cent price increase. Marvel released a 35 cent variant for every 30 cent comic they produced from June to Oct. 1977. This was a total of 184 different comics. These 35 cent variants were printed in much smaller numbers than the normal 30 cent counterpart and were believed to have been shipped to only six distribution locations to be sold in those specific areas.
Verpoorten credit per Nick Caputo, August 2012. Previous indexer credited Frank Giacoia, as did Marvel Comics Index #8A.
Letters from Frank Antonides, Bill Dickerson, P. Fenton Wagner, T. Gortel