So one other IRL application . . . pre-staged equipment. I want to say at one point in the Cold War the US had equipment staged at Diego Garcia for a larger armored formation. I know they had equipment staged in Saudi Arabia after Desert Storm so forces could rapidly re-deploy from stateside if the balloon went up.
As the Reunification was wrapping up and troops shifting from one theater to another, it would be easier to put primitive and early model mechs into storage and transfer the troops to meet up with the latest upgraded equipment as part of the refit & rest period before plunging into the fires of the theater they transferred to fight. When did the Star League convert to single-design battalions & regiments? Especially with the military-industrial complex theories about the SL procurement, they are not going to want the replaced mechs shipped back (remember Lord of War's line about selling M-16s by the pound!) since its all about the SL buying new stuff.
Heh, would observers really be able to notice that Wolverines, Shadow Hawks, Dervish, Archers, Thunderbolt, Riflemen, Wasps or Griffins were primitive? They would just write off the lesser performance IMO to being in the typical Periphery poor state of repair/operation . . . finding Icarus, BattleAxe, Alfar, Firebee, or Swordmen would raise way more questions.