What practical feature did Walter Schlage introduce around 1907 or 1908?

Answer

A key that operates from either side of the door (double-entry lock)

Schlage is credited with patenting the first practical lock that could be opened with a key from either side of the door—a double-entry lock, around 1907 or 1908.

What practical feature did Walter Schlage introduce around 1907 or 1908?
inventioninventorsecuritymechanismlock