If I remember correctly, this was how Eureka laid out the line up - I’ll go in order of more deluxe to more basic:
1680: Deluxe version had wide carrying handle, Cordaway cord reel, control panel with Bag Guard and dial suction selector, deluxe attachments (Vibra Beat rug nozzle, regular rug nozzle, wider upholstery nozzle, square horsehair dusting brush), maybe a stronger motor too.
1660: all of the above except regular Eureka attachments: no Vibra Beat, smaller standard upholstery nozzle, standard round dusting brush
1640: same as 1660, except smaller handle and no Control Panel with suction dial and Bag Guard - the only extra feature was the Cordaway cord reel...also a weaker motor I think.
1620 - base line model: same as 1640 with no Cordaway - as the calm voiceover in the commercial says “no fancy stuff, no fancy price. Just a sweet and simple cleaning machine...”
The feature distribution was similar for the previous Empress II FastVacs: 1880, 1860, 1840 and 1820....if I remember correctly.