After the beginning of the Atomic Age and before the first modern super-heroes started to appear, there was the Monster Age. It was an age where the Earth was under assault by all manner of strange, usually giant, creatures. Some were aliens, suddenly interested in Earth for the first time now that we had mastered nuclear energy. Others were ancient creatures awoken by our nuclear tests, or stirred to action by Deviants threatened by human's technological prowess. Others were the first mutates, created by nuclear radiation.

The Monster Age starts earlier than, but eventually runs concurrently to, the Hero Gap, and officially ends when the FF put a stop to the Mole Man in FF #1.

Tales To Astonish #13
Amazing Adult Fantasy #9
Journey Into Mystery #72
Strange Tales #75
Tales To Astonish #15
Tales To Astonish #25
Strange Tales #78 1st Groot 
Tales To Astonish #20
Tales To Astonish #17
Tales Of Suspense #18
Journey Into Mystery #65
Strange Tales #88
Tales Of Suspense #11  
Journey Into Mystery #63
Strange Tales #73
Tales Of Suspense #28
Strange Tales #89 1st Fin Fang Foom 
Mystery Tales #21
World of Fantasy #11
World of Fantasy #13
Tales of Suspense #9 origins of the Headmen 
Journey Into Mystery #68  
Tales To Astonish #25
Marvel Tales #113
Adventures Into Terror #28
Men's Adventures #26 1st Gorilla-Man 
Strange Tales #97  
Amazing Adventures #1
Astonishing #47 1st Dr. Druid 
Strange Tales #97
Amazing Adventures #3
Amazing Adult Fantasy #14
Marvel Tales #146  
Strange Tales #97
Tales to Astonish #21
Amazing Adult Fantasy #14  
Amazing Adventures #2
Strange Tales #97
Marvel Tales #134  
Journey Into Mystery #66
Tales to Astonish #25
Journey Into Mystery #79
Strange Tales #73
Tales to Astonish #23  
Marvel Universe #4-7 Formation of the Monster Hunters