How do Legacy raid lockouts work in World Of Warcraft?
Solution 1:
In legacy raids you have one shot at the mount per lockout.
You can clear the raid once per lockout, regardless of its difficulty (since you're able to change the difficulty while you're in the raid instance).
An exception are raids with the raid finder difficulty enabled (Dragon Soul and later). You can talk to the raid's npc and clear the raid finder difficulty as well. For Dragon Soul that guy stands right before the entrance, but since you can't loot the Experiment 12B on this difficulty it doesn't help you anyway.