Amazing preservation, I follow a guy on YouTube who collects and tests military MRE's, Steve1989MREInfo. Here is a list of his 1899-1945 sampling, needless to say 75 years ago and the packaging is still almost new. A month ago he posted the opening and tasting of a 1945 C Ration that if I recall correctly cost him roughly $3,000!
Steve1989 Ration Reviews (MRE) (1899-1945) - YouTube