You can see what they're getting at here. The idea is that the Micromasters will have an Optimus Prime like truck base which mirrors it's drivers vehicle mode. However they also decided that the large truck should be made of two vehicles that combine in a manner similar to Full-barrel & Overflow! Yes the truck splits down the middle of the longest side, between the second and third set of wheels to give you a front half made of the cab and the front of the trailer and a rear half made of the rest of the trailer.
Take the front half. Fold the trailer sides down as wings. Fold the wheels under the vehicle so the hubcaps face down (this stage isn't essential, but it's in the instructions). Fold the top of the vehicle over the back to form the nose of a jet. Turn the vehicle round so the shuttle nose is the front. Remove the exhaust pipes and re-attach pointing forward via the 5mm peg on their side as the jet mode's Scrambler Guns. Remove the missiles, turn them round and replace. Open the cab and, since this is the front half of the vehicle, sit Full-barrel in it. Now I'd have had half the panels to fill the V shaped gap on the truck's side attached to the jet's nose. They'd have folded out to the sides at this stage to provide forward wings.
Take the rear half of the trailer. Remove the laser machine guns. As per the front half,fold the wheels in under the vehicle so the hubcaps face down (again this stage is in the instructions but not essential). Fold the top forward over the join between the two halves of the vehicle to form the nose of a space shuttle. Fold the sides of the shuttle down as wings. Peg the guns into the wings and let Overflow take command of this half of the vehicle. Again as per the front half, I'd have the remainder of the panels to fill the gap on the truck's side attached as fore wings, a bit like Dirge's, to the nose of the shuttle.
We end up with two similar quite similar looking vehicles. The shuttle has a moulded nose with the crew riding in a sunken pit behind the cabin windows which has two Micromaster foot pegs in it as well as a moulded control panel at the front. The shuttle's cargo bay doors open, but the door sides won't fold flat unless the guns are removed or the wings angled down. What's revealed within the Cargo Bay is the split compartment we glimpsed when we opened the rear door of the truck. Each side has two Micromaster footpegs with the edges of the cargo bay doors each having two 5mm peg holes for you to mount guns in. The rear door still opens in this mode. If you've left the wheels folded down then the shuttle will happily roll along on them as will the jet. The jet's paler blue nose includes a fold up canopy to allow you to sit your Micromaster pilot inside. The body of the jet is dark blue and flat until it suddenly bulges up at the rear. And here's the real problem with the jet: IT'S GOT THE TRUCK CAB HANGING OFF THE BACK! No attempt has been made to disguise it, no joint so it can be folded down to give the jet a flat appearance, no engines or tail fins to be folded over it to disguise it. It's just there. I thought it looked awful when I played with one in Hamleys all those years ago, I think it looks awful today and it's quite possible the piece of Transformers I'm most embarrassed by. Well that and the Bay movies.
The two toys have one last trick to perform: they combine in vehicle mode too. Close the shuttle cargo bay doors. Match the large pegs on the roof of the cargo bay with the holes on the bottom of the jet, resulting in the jet resting on the shuttle in the same way that a real life space shuttle is transported by aircraft or Micromaster Skystalker's Air Unit rides on his shuttle.
The only thing I've not managed to mention that needs attention drawn to it is the lack of Micromaster ramp connectors. This is the only Micromaster "base" toy not to have them. Technically no part of this toy *is* a base but it would have been nice.
In 1990 the 17 year old me hated it. In 2010 the 37 year old me sees that the toys have some worthy play value but could be improved by filling the gap in the side of the trailer mode and loosing the frankly awful truck cab on the back of the jet. I can see the influence of the Action Master toy line, which was running parallel to the Micromaster combiners, in that we have a vehicle turning into another vehicle (vehicles in this case) both of which the figures supplied with the toy can play in.
The Micromaster Battlefield Headquarters, Full-barrel & Overflow were sold in the US and UK during 1990 and continued to be available in the UK at least during 1991. It's the last large toy released in the original Transformers line in the USA.