Flat HEDs (Heavy External Defense suits) were once the mainstay of the armored infantry of the most powerful countries from 2085 to 2144 CE, and are the predecessors of the SHELL combat frames used today. though not as modular, it was more agile, and due to its size required less power to run on. but as more powerful weapons became the norm, adaptability was key. but it simply could not be accomplished within this design. it could not take sniper roles supporting fire, nor could it be suitable against tanks or missile launchers. and when one part got damaged it cannot be replaced on the field. these concerns are what gave rise into the development of the modern SHELLs with their modular and interchangeable nature, retaining only the torso where the pilot sits and controls the machine, or as it's called~ the core. these days FlatHEDs are sold as army surplus to impoverished nations looking to up their defense (or offense) if only slightly. sold mostly to countries of lower economic capacity, or to insurgent forces, about 5 FlatHEDs equal one SHELL. it may not be much but it is still intimidating in its own right.
also called: Crabs, Hunchbacks, Bulletbox/es.
Scale Comparison with Saber shell:
I love Mechwarrior pilot figures =D