If it has a corporate model plate sticker on the bottom, you may get another clue: if it says Eureka Williams, then it is from before 1975. If it says "The Eureka Company", then it is from after 1975. And, if it says White Consolidated Industries, it is from after 1986.
Let us know what you find if you own this machine!
(Incidentally, the motor hood vents were rarely seen on Sanitaires - only on those with curved shaped 1960's style hoods. So the presence or absence of vents on this Sanitaire can't really help us date the machine....)