I think that 1963 date on the labels must be a mistake, or perhaps really applies to the sheet music publication, because wikipedia's article on the musical indicates that it debuted in Detroit in November 1963, moved to Broadway in 1964, and then this London production premiered in December 1965. Supporting the 1965 date is the upside-down matrix numbers starting with an S (much clearer in the Discogs entry), meaning that this must have been mastered in 1965, and looking at the back cover image in Discogs, I think there's a 1965 copyright date there, but the image is too small and blurry to be sure; the US, Canada, Germany and New Zealand entries at Discogs are all also dated 1965. Confirmation of the copyright date on the back cover (just above Mary Martin's photo) would be good, but this is looking to be changed back to 1965.