Exactly as it says above. Basically these consist of tiny changes to canon designs, not even worth full stats, but worth the story they tell.
Marauder:
The 3R variant of the Marauder is so ubiquitous that it's easy to assume that it's actually the original version. Hell that Valiant Lamellor armor by itself caused endless debates among the mech enthusiast community over whether or not it was a ferro fibrous compound, some sort or high end warship plate, or some other form of lostech super armor (long story short, it's pretty mundane name brand standard plate used on a bunch of designs, and Comstar needed to do more fact checking).
Anyway, the 3R standard was actually a design standard developed by General Motors of Kathil once more advanced supplies ran dry, and involved replacing the original advanced armor with a more mundane type (the now famous Valiant Lamellor), and redesigning the ammunition feed system. This became the go-to template for all future production Marauders until the recovery of advanced technology, to the point where you could generally assume that if it was a Marauder, and had two PPCs, two lasers, and an autocannon, you could assume it was a 3R, and if it didn't carry that weapon load, it was at least derived from the 3R.
There were exceptions.
The original 1R Marauder, which served as the basis for the GM redesign, had one additional difference aside from the armor and advanced ammo storage system. The munitions bay was on the other side, basically directly under the cannon. When they redesigned the ammo bay, GM attempted to replicate the placement, however the less sophisticated design suffered jamming to what was deemed an unacceptable degree. Moving the bay gave them plenty of room to work and prevent the jamming problem, and by the time anybody realized how terribly prone to explosion that made the mech, the design was already locked in. When GM realized the problem, they shrugged their shoulders and turned out the 3D variant as an alternative.
Even so, it took a few decades before the 3R became the de-facto standard, and before that many a catastrophically damaged 1R (and 2R) marauder was rebuilt by their pilots with the ammo bay in its original position. Jamming problems aside, many of these Marauder pilots grew to be thankful for the extra protection this little idiosyncrasy provided.
Though we can't consider GM completely negligent. Despite our previous statement GM produced a minor change of their own to the design to mitigate the risk of explosion on the field by wrapping the ammo (and the feed leading into the autocannon) in the cooling sytem. Unfortunately, this managed to cause more problems than it solved, and had to be abandoned, with only a few examples in the field.
These variants were always relatively rare, and would only become more so as time wore on. Technicians in general preferred to work with a factory designed and approved blueprint rather than a quirky homebrewed design, so critically damaged Marauders were usually rebuilt to the 3R (or one of its variant) standard regardless of their original configuration. The so called "ammo variant" seemed to undergo a resurgence with the appearance of Wolf's Dragoons in the Inner Sphere, though it eventually became apparent that their Marauders were mostly actual 1Rs, complete with CASE, with the advanced armor stripped off. Even the Dragoons, however, tended to go with the 3R standard when repairing a crippled Marauder.
There might be an example or two of these variants kicking around here or there in the modern day, though since none of these variants ever recieved an official designation, it's impossible to be sure.
Game Notes
*The MAD-3R Marauder (Ammo Variant) uses the same stats as the standard MAD-3R Marauder, with the exception that the AC-5 ammo is located in the Right Torso. This variant recieves the additional design quirk Ammunition Feed Problem (1)
*The MAD-3R Marauder (Cooling Variant) use the same stats as the standard MAD-3R Marauder, with the exception that a single heatsink is removed from each leg and added to each side torso. This variant recieves the additional design quirk Cooling System Flaw (3)
*The MAD-3R Marauder (Dragoons Variant) uses the same stats as the MAD-1R Marauder, with the exception that the mech uses standard armor. This reduces the armor level to the following: 176 overall, H:9 CT:33/10 RT/LT:18/8 RA/LA:19 RL/LL:17
(more to come. At the very least a wasp with one hand...)
