Nightmare #6 was published by Page Publications c.1977.
The contents are the same as Nightmare #15 published by Skywald:
The only difference in the contents is that the stories are published in a different sequence, and the Australian edition does not include the letter column A Wretched Bunch of Letters. Presumably there are advertisements in the Skywald edition not replicated in the Page edition.
The other difference, of course, is the covers on the respective issues. What is especially noteworthy is that the cover to the Skywald issue was published by Page Publications at least once - as #8 in the same series:
This cover scan is taken from the AusReprints listing.
As you can see from the AusReprints cover gallery for this series, there is no image for Nightmare #5. My collection of such issues is rather patchy, and unfortunately I don't have a copy of Nightmare #5, nor of Nightmare #8 . I've asked a few likely collectors for information, and have come up dry.
So let me speculate and pose some questions:
1. I think we'll find that Nightmare #5 has the same cover as Nightmare #8
2. I think the contents will be the same as Nightmare #6
3. If this is correct, the new cover to #6 was rather devious in its intent
4. But, where did the cover image for Nightmare #6 come from? A European source?
5. And wouldn't it be even stranger if Nightmare #5 didn't exist? Unlikely, but it would put a different spin on the cover to Nightmare #6.
6. Is there a third iteration of Nightmare #8 ie. an unnumbered issue? Or an unnumbered edition of Nightmare #6?
I look forward to answering these very important questions.