The Army decided, long ago, that comic books (or at least comic book format) was a good way to get things over to soldiers. They hired Will Eisner (a very good artist - the Eisner Awards are named after him) to do them. Manga hare not the only instructional comics! (Hell, I've even done instructional
comics myself.)
I'm pretty sure the rifle maintenance booklet was done by Eisner, or at least his shop. The battery maintenance one was definitely not done by Eisner - the artist may have done work for Archie comics. The Gama Goat one, it's a tossup - whoever it was was trying for the Eisner style, but it's not the Real Thing.
I was told by Mort Weisinger (who was the editor of Superman at the time) that during WWII editions of Superman were published in Basic English, to help bring the less-literate recruits up to speed.
Behold the power of comics and manga!