That means they were cast in September/October 1963. She would have been assembled within a couple of months of those dates. So your "D" is most likely a "C" making her a 1963 tractor.
I have rechecked my data and realised I made an error. The numbering is rather confusing and they went back to 90 in January 1964 after reaching 934XXX in December of 1963. So your tractor is a Novenber 1963 which would line up with the casting codes.