Hi Bob,
Another giveaway is the 65-66 jacks are shorter when compared to later versions when full extended. The 65-66 is 16 inches tall when fully extended. The second generations 67 to end of 70 was 17.5 inches tall. See article "Ford Mustang Jacks" by Marcus Anghel and Bruce Klier, dated January 2013. It is a PDF. There are great images showing the many difference.
Cory