"Prisoners of the Mysterious Master Mold!"
The team tries to infiltrate the base of the Sentinels as Hank and Bobby are taken captive. Meanwhile, Trask is brought before the ultimate Sentinel, Master Mold and given an ultimatum to help Master Mold create new Sentinels. Master Mold studies his prisoners and learns the origins of Hank McCoy until Xavier cuts him off just before he reveals the secret base of the X-Men. But the X-Men cannot stop Master Mold as he continues to threaten Trask into helping him create new Sentinels.
First appearance of the Master Mold. Origin of the Beast.
Story and cover reprinted in X-Men (1963) #68.