A little more of the back-story - this is how I've incorporated the design into canon (MTU is pretty faithful to canon's story line):
The first incarnation of the hybrid heavy fighting vehicles for cavalry (combat) formations was locally produced for colonial formations, but the success of the vehicle prompted Imperial procurement officials to adopt the design. The original concept had three vehicles designed at TL11, one with a mass driver (the M133), one with a plasma gun (the M135), and one with a HVEL gun (the M134). Later upgrades to these initial designs provided point defense capability (the A1 version) and improved weapons, performance, and armor (A2 through A4 versions). There are still examples of the M133, M134, and M135 A4 versions in service with colonial formations across the Imperium (produced at TL 13).
The next update to the design required a redesign of the entire hull, so it was no longer possible to retrofit older hulls to upgrade them to the A5 variant. With the longer range available in energy weapons at this time, there was no longer a need for the HVEL cannon on the M134, so only the M133A5 and M135A5 were produced. The A5 series (TL14) was fielded prior to the 5th Frontier War and these vehicles are now (in 1112) being systematically upgraded to A6 specifications.
The first A6 models reached the Spinward Marches in 1109. The first unit to receive the new models was the 322nd Cavalry (Combat) Regiment, home-based on Rhylanor (Spinward Marches, 2716). The 322nd employed the A6 variants in the 5th Frontier War, with the regiment’s 3rd Squadron against the Sword Worlds forces, and with 1st and 2nd Squadrons, committed against the Zhodani on Esalin. The A6 models also outfitted some of the Imperial Army Regiments dispatched from Capital (Core, 2118) for the defense of Rhylanor but arriving in 1109 after the Zhodani had been thrown back by the Corridor Fleet. These formations also fought the Zhodani in various locations in the Jewell Subsector.
