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What was happening in 1977 for Def Leppard?
Rick Savage, Tony Kenning, and Pete Willis, all students at Tapton School in Sheffield, South Yorkshire, formed a band called Atomic Mass in 1977.
Did Atomic Mass turn into Def Leppard?
Joe Elliott proposed the name Deaf Leopard which was originally a band name he thought up while writing reviews for imaginary rock bands in his English class.
Why did Atomic Mass change their name to Def Leppard?

Question: What year did the name change happen?
Self-contained rewrite: What year did the name change from Atomic Mass to Def Leppard happen?