I remember buying that one off the stand, as well. I remember even as a kid being puzzled by the newsprint cover. Maybe a failed experiment by Gold Key? There's a similar 25 cent reprint issue from that period that reprints "The Old Castle's Secret" and another story or stories. The cover is printed on regular cover stock, though.
You are referring to the Best of Donald Duck and Uncle Scrooge #1, which reprints "FC #189 - Old Castle's Secret" and "FC #408 The Golden Helmet." Issue #2 reprinted "FC #256 - Luck of the North" and "US #7 - Seven Cities of Cibola." The cover of issue #1was modified to include Uncle Scrooge, whereas the original only had Donald and the nephews. In my poor student days, I bought these reprints before going after the originals just so that I could read the stories.
I never noticed the modified cover. Is it known whether Barks redrew it? Old Castle's Secret and Golden Helmet were probably my two favorite duck stories as a kid. A couple of years later I picked up a really beat copy of FC 422, Gilded Man, which is also a great favorite.
The new cover was penciled by Tony Strobl, according to http://coa.inducks.org/issue.php?c=us%2FBDDUS+1