Exterminator EXT-4D
Current: SLDF, Royal Divisions, RWA/AEAF Terran Corps
Suggested: IS General, SLDF, Royal Divisions, RWA/AEAF Terran Corps
Why? The EXT-4D came out in 2620. 10 years later, the stealth version (EXT-4C) came out. The -4D has no restricted technologies whatsoever. The XLE and AMS were pieces of equipment on units which were sold as SLDF surplus to Member States (and the Periphery). It doesn't even have a Guardian ECM Suite or Active Probe.
Seems to me like the EXT-4D was the "consumer-grade" model, sold off to prospective buyers, while the EXT-4C and SPR-4F were kept as Royal-exclusive, fulfilling the mission of "seek-and-destroy". That would explain its appearance in Great House RATs and the sheer numbers surviving into the early Succession Wars. Had it been SLDF / HAF exclusive, most (if not all) of the advanced units (i.e: Devastator, Nightstar, Spector, etc.) would have been destroyed during the drive for Terra or evacuated on the Exodus. The standard models (EXT-4D) would probably be left behind; and they were.