【星野金属の】ソルダム 骸形化4回目【教訓は?】

A muscleback iron is the term given to any iron in which
there is no cavity on the back of the head, i.e.,
the weight is more evenly distributed across the back of the clubhead.

ヘッドの後ろに空洞のない、クラブヘッドの後ろにも均一に重量のあるアイアンのこと。