Checking the Scorpion Empire MUL mech list for an Omega force, i came upon the Naja, a very interesting mech from TRO:GC. A very old CCC mech, it was last avaible during the Clan Invasion, but looks like that both the Naja and its predecesor, the Kintaro -19b-EC, are avible to the SE since the Late republic Era, with only the Naja going so far to the IlClan Era.
Nasty medium for second line forces, and even uses "improved" SRM systems, a step between the IS and Clan missile systems.