Distribution date from Joseph Marek's Marvel Comics Group history website.
Tied with Patsy Walker (Marvel, 1945 series) #95 (on sale date the same) as first use of MC brand, which is considered to be the first use of the company being called Marvel Comics in the 1960s.
On sale date from the publication date reported to the U.S. Copyright Office found in the Catalog of Copyright Entries, Third Series, Volume 15, Part 2, Number 1, Periodicals, January-June 1961. Class B periodical. Copyright registration number B895314.