Bossk was advertised on packaging as a secret action figure from The Empire Strikes Back. Nobody knew at first what the figure was going to be. Later on during the promotion he was revealed by a sticker placed on the packaging and also a T.V. commercial. The offer required 4 proofs of purchases and expired on May 31st, 1980. Bossk is considered actually the hardest of the mail-away figures to find with the original box, figure, and paperwork. He included a flyer like the Boba Fett figure describing his role in the movie which is pretty impressive considering you actually only see him for probably half a minute on screen. Here is how the offer appears on cardbacks
Here is my Bossk with the box, figure, and paperwork.
They flyer included reads:
“Bossk is an alien bounty hunter — one of many commishioned by Darth Vader to seek out the Rebel Alliance. Darth summons Bossk, Boba Fett, and other strange bounty hunters (including a droid) to his command ship, Executor. Here Darth orders them to find the Millennium Falcon containing Han, Leia, Chewie, and C-3PO as they attempt to escape Darth’s vengeance.
Look for Bossk and more exiting action in the next Star Wars movie: “The Empire Strikes Back” this May.”
Finally here is the commercial which advertised the promotion.