I don't know if I agree with the above. You really need to think about what kind of a campaigns the Successor States were planning at that time. What makes an AAA mech unique? Is it that they have LBX autocannons or long range cannons? What do they provide to my limited mech assets besides Air Cover against the rare air attack that should have been dealt with ahead of time.
Also what kinds of Aerofighters are we dealing with here? I don't think it has ever been stated what kinds of Aerofighters are most commonly encounter on a mission. Light ones like the Cheetah and Thrush, Mediums like a Lancer or Stingray, or Heavies like the Shiva or Chippewa?
Each can be countered differently
Lights can be swatted out of the sky with LRMs which losts of mechs have
Mediums can lay down some hurt but an ERPPC or Gauss Rifle will mess up their day, lots of mechs have either
For a Heavy you need insane amounts of standoff and/or clusters to force a control roll and that is perhaps the only one that would require dedicated AAA in the form of an Arrow IV or Flak artillery because you don't want that to get near you
AA guns are good for more than just ASF. LBX pellets or light AA flak shells are probably the best way to swat VTOLs, and a good way to attack anything else airborne, like WiGE craft or even air-dropping mechs (not something you'd get a chance to fight every day, I realize). Flak shells also do full damage to infantry, so you can actually use light AC shells as long-ranged anti-infantry fire, though it's not a great use.
I think that there are a couple out there but most are variants of line Mechs and they aren't fluffed as anti-air mechs.
3050 - Cataphract, Annihilator
3055 - Daikyu (LBX 5 Variant)
3058 - Longbow (one with 6 Light AC5), Emperor
3067 - Vanquisher (Either the 4 Ultra AC5 version or Rotary 5 version)
Ultra5s and RAC5s aren't good AA guns. Their range isn't great, and they don't get a flak bonus. LAC5s also have pretty poor range, but at least get a flak bonus. LBX10s are ok, they do get a flak bonus, but again their range isn't anything to write home about, and range is maybe the most important thing in ground-to-air fighting. Another thing to consider, though, is quirks: The Rifleman and JagerMech both have the Anti-Air Targeting Quirk, providing an extra -2 against airborne targets. That's not something very many mechs have. Can anything with an LBX-2 or -5 do AA duty? Yes, they're still getting the flak bonus. Is it going to be as good as a mech with the specialized AA Targeting Quirk? No, it's not. There are actually more mechs with that quirk than I thought, I should say. I just checked the BattleMech Manual, and it lists the following:
Deimos, Galahad, Hatchetman, JagerMech, JagerMech III, Lancelot, Longbow, Orion, Orion IIC, Perseus, Rifleman Rifleman II, Rifleman IIC
Of those, only the Deimos, Hatchetman, JagerMech, JagerMech III, Lancelot, Longbow, Orion, Perseus, Rifleman, Rifleman II, and Rifleman IIC have any variant that mounts a flak-capable weapon. Those are going to be the best options, since they get the flak -2 and the AAT -2.