Assassin II (MekTek)
This BattleMech was published in an official BattleTech product, namely the MekPak expansion for the MechWarrior IV: Mercenaries computer game. However, although official, computer games are not considered among the sources for Canon unless the information is expressly confirmed as canonical elsewhere.
By virtue of being an official albeit not positively canonical product, the game expansion and the 'Mechs presented therein including this one must be considered apocryphal.
The Assassin II is the result of a Federated Suns specification calling for a "quick strike harasser unit". The Maltex Corporation proposed a major redesign of their older Assassin chassis, the resulting BattleMech exceeded all expectations and has started to see use as a counter to the stealth technology employed by the Capellan Confederation.
MechWarrior IV: Mercenaries MekTek MekPak Expansion