From the back of the book:
"In the 3050's, the discoveries of long-buried Star League records led to a plethora of new cutting-edge BattleMech and vehicle designs by the end of that decade, resurrected from the blue prints of design firms centuries old and thought lost to antiquity. Now, ten years later, these designs have moved from novelty to workhorse, as the Inner Sphere once more finds itself at war!
Fully updated to circa 3070, Classic BattleTech Technical Readout: 3058 Upgrade launches these designs into the current time frame, including notable MechWarriors/crews, as well as new variants. Additionally, a battle armor section has been included, providing full Classic BattleTech stats, and updating the histories for these twenty venerable designs."