That may not be true in every case. Though I did not research all the magazines, I found that Umbrella from the 13th has a Vampirella counterpart with a very similar cover. That issue is from 1973 or 74, but remember that all the magazines from the later series were originally planned to be part of an all-magazine 11th series. While a bunch were subsequently pushed back into the 13th, 14th and 15th, all issued 1975, the associated parodies were likely created at least a year earlier.
Conversely, I could not find a real-product counterpart matching Vague magazine from Old School 6, so I too wonder if some of the magazine parodies are completely imagined. I don't recall Dave ever posting the real product images from OS6 as he normally has on prior series; I'd be interested to see if Vague was a close match to the real product used.
If you, or anyone else, want to find out more about the unmade Wacky Magazines series and see images of them, go to lostwackys.com and you'll find a whole section on them. I hope this helps you out.
UPDATE: Here's the magazine titles that were intended to be in the 11th series, but were dropped:
*Tomb Magazine (Time Magazine, released in OLDS 5),
*Rotting Zone (Rolling Stone, still unreleased (hopefully that'll change, someday)),
*Schnozmopolitan (Cosmopolitan, released in 1982 Album series, FB 1, and ANS7 (the last one as a foil sticker).
Now here's the unmade Wackys from the magazine series that I've seen in the aforementioned section:
*Creep (Creem),
*Eater's Digest (Reader's Digest),
*Foodoplay (Photoplay),
*Grumpy Dumpty's Magazine (Humpty Dumpty's Magazine),
*HenHouse (Penthouse),
*Madamgazelle (Mademoiselle),
*Roman's Day (Woman's Day),
*Salami Street Magazine (Sesame Street Magazine),
*Song Hates Magazine (Song Hits Magazine),
*True Confetti (True Confessions),
*Zit Parader (Hit Parader).
Those are just the ones that I've seen on the website. Who knows how many other roughs for this unmade series may be out there, still undiscovered?
I also think that any one of these would be perfect for future OLDS releases. What do you guys think?