>>5305659They do have gearboxes, good eye!
But I'm pretty sure the things you're referring to are the "roll cages."
The exposed balljoints on standard CCBS torsos provide very decent hardpoints for attaching things, so I designed a technic assembly that clips on to fill space, add some "industrial" detail, and provide extra surface for greebles and details.
The MTron mech has a gundam-ass "back-shield" to protect its chassis, while the Ice Planet mech has an "ore storage" pod.
Just waiting on an LDD order to come in and I can build these IRL