Could be, 64 was a half model year. My car was built in July 64. Haven't looked into the Signet yet, but its a 65.
Bottom line is there were a few 64's built with a 22 inch radiator. I would have to say that the 65 Signet I have is a mystery because it started out life as a 170 /6 automatic non-a/c car, It has a 22 inch radiator in it too, not original but it bolts right up.