This middle pedal of this 1915 Stieff upright operates a muffler rail which drops when the pedal is depressed, there is no spring on the rail as in modern pianos. When the pedal is depressed, a spring loaded latch swings over the pedal and holds it down. However, there is no (apparent) mechanism to release the latch and allow the pedal to return. It seems likely that further depressing the pedal would have been the means, so the question is "how?".