I noticed it years ago, when I first started recognizing cover artist styles. It is one of the rare cases Curt signed anything.
Here is the cover in quesiton (it's a little hard to read in this scan):
Too bad they messed up the cover when they reprinted it in Supergirl Archives 2 (using a redrawn Supergirl head from a reprint).
And here's something else I just ran across: Curt's National Cartoonist Society biography (click for a bigger version):