Transformers that don't transform. Action Masters are remembered (if they're remembered at all) for this basic fact. Whether one considers this the basis for disdain for Action Masters or not, or accuses Action Masters of being the reason for the end of the original American Transformers line (as some do), it is pretty much undeniable that the robots in the Action Masters line do not transform.
However, what is not so well acknowledged, and really ought to be, is that all Action Masters were nonetheless sold with vehicles or weapons that do transform.* In the case of Optimus Prime, this meant the largest accessory in the Action Masters line, the Armored Convoy.
Since Optimus Prime traditionally turns into a truck with a trailer, the Armored Convoy is a fitting companion piece. A minor historical note: this is actually the first time that a long-nose cab is used for an Optimus Prime figure (such an alt-mode would be used for the Japanese-exclusive "Star Convoy" the following year, but not for an American release of Optimus Prime until the Generation Two line).
Just as most (though not all) Optimus Prime truck cabs separate from their trailers, the cab on the Armored Convoy separates from its trailer. The cab transforms into a "space jet" (don't ask me how a jet is supposed to operate in space. Maybe the wings are just for re-entry, like on the old space shuttles), with the "smokestacks" becoming weapons. An Action Master figure can sit in the cockpit, or (as seen here) stand just behind it, using the holes in the figure's feet and pegs on the vehicle to secure the figure into position.
The trailer portion opens up to become the "battle headquarters," which can accommodate quite a few Action Master figures in seats and pegged platforms located throughout. It all makes for a pretty fun set, and I really think that fans who avoid it just because the robots themselves don't transform are missing out. In fact, my only real complaint about this set is the fact that Hasbro decided to use that bright orange plastic for most of the set's weapons (and the smokestacks). I would have preferred that all that orange plastic have been done in black, instead.
But they didn't ask me....
*Well, with the exception of the European/Australasian Action Master Elites, but those are Action Masters that do transform (as odd as that may sound), so the point remains that these toys all retain some transforming element.