In the mission:supply under the salvage special rule they mention that 2 buildings contain a battlemech that must be rolled on the reinforcement table p69 but the table doesn't give you what dice you roll nor the chances to get x or y entry.
On the same mission, the player can take the option named Reinforcement but if I read it right it seems to help the attacker more then make the mission harder.
Reinforcements: The Defender may designate one unit from his force as reinforcement. That unit enters the battle from the Defender’s home edge during the Movement Phase after Turn 5 but before Turn 9 (player’s choice). If the Defender is destroyed before the reinforcement enters, then the reinforcement counts as a destroyed unit for determining objectives.
