Tetsujin 28-gō, the famous Japanese robot manga series written and illustrated by Mitsuteru Yokoyama, ran as a monthly insert in Shōnen Magazine from July 1956 to May 1966. The manga follows the adventures of a 10-year-old boy named Shotarō and his giant crime-fighting robot named Tetsujin 28 (a.k.a "Gigantor" in the US), which was originally built by the boy's late father as a secret weapon for the Japanese military during World War II.

Here are some scans of the manga covers (from the mook Yokoyama Mitsuteru Manga Taizen).

Tetsujin 28 manga cover --
July 1956

Tetsujin 28 manga cover art -- Tetsujin 28 manga cover --
September 1956 [+] // October 1956 [+]

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
February 1957 [+] // April 1957 [+]

Tetsujin 28 manga cover art -- Tetsujin 28 manga cover --
August 1957 [+] // October 1957 [+]

Gigantor manga cover --

Gigantor comic cover art -- Tetsujin 28 manga cover --
November 1957 [+] // December 1957 [+]

Tetsujin-gō 28 manga cover -- Tetsujin 28 manga cover --
April 1958 [+] // November 1958 [+]

Tetsujin 28-go manga cover -- Tetsujin 28 manga cover --
June 1958 [+] // December 1958 [+]

Iron Man 28 manga cover -- Tetsujin 28 manga cover --
March 1959 [+] // July 1959 [+]

Tetsujin 28 manga cover art --

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
August 1959 [+] // September 1959 [+]

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
April 1960 [+] // June 1960 [+]

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
July 1960 [+] // August 1960 [+]

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
October 1960 [+] // November 1960 [+]

Tetsujin 28 manga cover --
December 1960

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
March 1961 [+] // August 1961 [+]

Tetsujin 28 manga cover --
November 1961

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
January 1962 [+] // May 1962 [+]

Tetsujin 28 manga cover --
February 1962

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
September 1962 [+] // December 1962 [+]

Tetsujin 28 manga cover --
August 1962

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
February 1963 [+] // April 1963 [+]

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
November 1963 [+] // December 1963 [+]

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
January 1964 [+] // July 1964 [+]

Tetsujin 28 manga cover --

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
January 1965 [+] // March 1965 [+]

Tetsujin 28 manga cover -- Tetsujin 28 manga cover --
September 1965 [+] // October 1965 [+]