Bryan,
Page 24 of Marti's Tagbook explains the transition. Before 12/28/1964 Mustangs were painted with the older enamel. After that they transitioned to Acrylic Enamel. To identify the difference, the warranty door tags used different colored backgrounds: black for the older enamel and gray for the acrylic enamel. Then in 1966 they switched this and the black background meant acrylic enamel.