Carl Barks' Surviving Comic Book Covers

Carl Barks drew 256 Disney comic book covers during the years 1942-69. There exists only about 21 pcs original artwork of those comic book covers.
Which are those surviving original covers and how they survived?
As known, the publisher trashed and burnt almost 97% of all Carl Barks' comic book original artwork, consisting some 6700 pages all together, so there exists only circa 200 pages of original comic book art.