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It's my understanding (not having got my copy yet) that - leaving aside the title - the story has absolutely nothing to do with the Martian Manhunter.

No, but it is a Prototype story, just as Supergirl and other characters have had. The concept of a Martian policeman coming to Earth (even wearing an X-harness) was likely the germ of the idea, that was then developed into J'onn J'onzz two years later.

Several months ago, when the idea of an MM Showcase was being discussed on here, Betty and others were begging for it to be included.