You know those candies that you bring with you on road trips that are barrel-shaped? Yes, the root beer actually tasted like a root beer barrel which as you know, has always been a unique flavor of its own, apart from the root beer drink itself.
Also, these stills of them were taken right off another photo from the 21st of August, 1967 when Jimi and the band were at an airport in London. Also, the date does not add up.