In the Marine Corps, ' grunt ' is 'originally a pejorative term for Infantry Marines but now a source of pride'. 'Grunts of the 1/5' are Marine grunts belonging to the 1st Division and 5th Batallion (if I have my units correct) of the Corps.
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KeroroTo be preciouse, what is the difference between "young marines of the 1/5" and "yound marine grunts"?The author is comparing the young Marines of the 1/5 with young Marines in general. He is saying that the young Marines of the 1/5 are about equal in "hardness" with any other young Marine.