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Since they had already designed the Mk2 chassis for the Erets, Olympiadica Manufacturing top brass thought it wise and financially lucrative to forgo any updates to the "Anvil" beyond simply swapping the turret to the new Mk2 chassis. Fortunately for their bank accounts, insufficient employees of the government were observant enough to notice, and so the cash continued to flow.
Keeping with their unofficial company model, "saved money when sown together still won't work as an umbrella when the economy collapses," the engineers updated the chassis with a more all-terrain friendly track system without compromising the heavily armor or moving the front housed engine. Thus the Erets Mk2-a is still capable of both melting the armor off opposing mechs, until it runs out of power, that is.