I have no idea--it never happened. Of all the CIs that never were, The Mask of Red Death may be the one that most deserved to happen, if only because CI had such a way with Poe. But CI was moving away from horror about the time they made the above announcement (or away from horror covers, anyway), so it's highly possible that Mask got penciled out. Darn. If only they'd gotten to it a few years earlier. What would a Henry C. Kiefer Red Death cover have looked like? Awesome, I'm guessing.
But 1949 was the year that CI redid the covers to Dr. Jekyll and Mr. Hyde, Twenty Years After, and The Hunchback of Notre Dame, making them less gruesome (or, in the case of Hyde and Hunchback, less hilarious). And how were they going to do a non-horror Mask of Red Death cover? Would that even be possible?
And how were they going to soften passages like, "No pestilence had ever been so fatal, or so hideous. Blood was its Avator and its seal--the redness and the horror of blood"? Paraphrase it as, "The Red Death was bad news"?
Maybe they were planning to issue Mask with The Gold Bug, and then they scrubbed Mask, and Bug became the lead Poe story in the issue to come, if only because it didn't require horrific cover art. Maybe I'm babbling. In any event, The Gold Bug was still two years away when this "Look What's Coming!" announcement appeared.
In a just universe, a CI Mask of Red Death would have happened. Oh, well. No point obsessing over it (though that's never stopped me).
The 1949 Toilers of the Sea featured gorgeous art by August M. Froehlich, though the adaptation left out a few too many story details, in my opinion (a problem corrected in the 1962 revamp):