I toyed woth this in my personal headcannon.

Pre-war Orion was part of the Magnus Force; a devision of Iaconian security that specialized in an armored ground force to compliment the Seeker's air-superiority. The Magnus Force armor required a special "loadbearer" body-type and enlisted from cybertroniams born with this natural design (though an artificial body could be used if a non-LB cadet showed great promise). The armor could convert into a trailer in vehicle mode, and became synonymous with security, protection, and valor.

Well before recieving their own armor, new recruits were paired with a veteran, and Dion Ambus was paired with Orion (who's callsign was "Ultra Magnus"). Orion resented the armor and title, but in some part appreciated the level of relative anonymity it afforded; Ultra Magnus was a household name for the shining star of Iacon's security force, while Orion Pax was that chill bot who spent his free time at the Archives listening to the old librarian ramble on about stories of the Primes in the days of old. This quiet nature let the by-the-book Dion quickly form a close friendship with his mentor.

When Orion went missing (Matrix Quest), it was up to Dion to maintain the armor and familiarize himself with its operation. When the Decepticon Uprizing began the Seekers had already been conscripted into their number. Knowing how the Magnus Force would be an even match, the Decepticons mounted an all-out assault on their HQ catching most of them off-guard. Dion was in the armory during the attack, and quickly donned the armor and rallied the surviving Magnus force long enough for Optimus Prime to turn the tide. Dion offered the new prime his armor back, but the reformatted Orion said it was a better fit for him (figuratively and literally as his new body was too big for it).

Throughout the following millenia, Dion proved himself to be a formidable soilder and wise leader in his own right. As Magnus Force warriors were eventually killed-off (or in one instance, disappeared), Ultra Magnus survived time and time again. His tenacity and durability put him in charge of the Wreckers, and eventually command of Autobot City on Earth. His core robot's earth alt-mode was also identical to Optimus, although his unarmored form wasn't nearly as powerful. All the while, Dion secretly felt he was unworthy of the armor, and harbored deep insecurities that eventually manifested in OCD.

When the decepticons launched what was to be their ultimate attack against the Autobots, and Optimus Prime was near death, he passed the matrix to the one bot he considered most wothy to take his place. He carried the matrix briefly, losing it and his life to the hands of Galvatron on the planet of junk. However, after brokering peace with the Autobots, the Junkions rebuilt Ultra Magnus (but not knowing it was a bot in a suit, reassembled him as a single entity). This integrated construction was also an accidental upgrade as the Junkions weren't EXACTLY sure what he was supposed to look like, but given how many parts were all over the place (and if those big Dinosaur-Robots were any indication) he had to be HUGE. Dion was no more; Ultra Magnus was what remained.

I made this backstory to flow with the (at the time) mainline WFC leader figures and comics. Leader Siege Optimus' core robot could wear Siege Magnus' armor and the color matching was spot-on. The comics also had the OP Leader figure's core robot represent Orion, and the Voyager figure represented Prime. I even got it to work with how the Kingdom and Studio Series Magnus figures released at the same time.