Hello,
The term "Generations" of OmniMechs isn't a specific, easily defined term and can vary from Clan to Clan. Much like versions of a computer program or model years of a car, it's easiest to look at specific versions.
Most early OmniMechs, due to design difficulties with OmniMech technology or limited resources did not utilized all 3 of the advanced weight saving technologies (XL engines, Endo Steel internal structure, or Ferro-Fibrous armor). The Coyotl used Endo and an XL engine. The Lupus used Endo and Ferro. The Woodsman only used Endo. The Nova only used an XL engine. And the Crossbow (which debuted the same year as the Nova) didn't use any weight of the saving technologies. Later OmniMechs tended to use at least 2 of the weight saving technologies, if not all three.
Some clans produced more advanced OmniMechs from the start while others continued producing less advanced OmniMechs.
As for designs specifically mentioned as "second generation OmniMechs", if you read TRO3050 Upgrade, the Stormcrow, Mad Dog, and Hellbringer are mentioned as "second generation OmniMechs" somewhere in the TRO entry. The "Capabilities" section of the Hellbringer TRO entry begins by giving another descriptor of "second generation OmniMechs":
As with many “second generation” OmniMechs, the Loki shares molds and structural components of other Omni designs like the Thor.
Going by the TRO artwork, the Hellbringer and Summoner share similar designs of torsos, legs, and arms. The Mad Dog and Timber Wolf share similar designs to their legs, hips, and arms. The Gladiator and Gargoyle share legs, as do the Warhawk and Dire Wolf. A key feature of the design and manufacture of the Cephalus, Septicemia (Pariah), and Osteon is the similar body structures.
While the creation of Battle Armor handholds, power/comms connections, and gyro adaptations was a big step forward in OmniMech capabilities, it seems to be something that was retrofitted onto older designs and not necessarily the defining point between generations of OmniMechs.
