Rumor has it that only 100 of these were made, and only 15-30 are accounted for. These were never sold to the public.
Two major variants exist: an "L"-slot rocket firing mechanism and a "J"-slot mechanism, based on the backwards shape of those letters.
Almost all of the prototypes are unpainted, blue figures. One version is known to have yellow legs.
The "L"-slot missiles have 4 distinctive "ribs," whereas the "J"-slot version has 8 distinctive ribs. Since this projectile separated from the figure, it's not necessarily paired with every prototype; some were lost.
None of the figures have a copyright stamp on the leg, like the regular release of this figure (which did not have a working, firing rocket).
Each prototype figure was loose, however, one version was shown carded at a Toy Fair expo.
Tomart's Action Figure Digest #20 has an authoritative write-up on the further back story.
* Rocket Firing Boba Fett (L-Slot Version) First Shot (TheSWCA.com)
* J-Slot Boba Fett (TheSWCA.com)
* Rocket Fett analysis by Chris Georgoulias (12Back.com)