Just checked my old C64 original disk box of Barbarian, & definitely on the reverse side of the cover. Wow, I even still had the poster!
Of course I still don't know if that'll be the case with the Amiga version,but its been a fun history lesson!
It seemed a lot of Palace games did that at the time, same deal with a graphic adventure/arcade game called Stifflip & Co. too.
Always annoyed me I finished 89% of Part A of Stifflip, & never knew how to solve Side B "Epilogue"!