Might it be an attempt cause greater hammer shank acceleration, and, to mimick the repetition abilities of a grand piano? In other words, make the repetition of the action faster like a grand piano?
This presupposes that there was a problem with hammer speed in the first place (it seems safe to assume that some problem existed that justified the attempt at improvement).
If the hammer was not departing the top of the jack at sufficient speed, the top of the jack would next strike the metal wire sticking out of the green felt/button. This might have been designed to impart greater acceleration to the hammer. The little curly Q tip of that wire suggests it was intended to receive a blow from the direction of the top of the jack, with the curved tip guiding the jack toward the other end of the wire.
And, the bottom surface of the "button" at the bottom of the mystery device appears to be slightly lower than the bottom of the hammer butt leather. If the jack could be caused to travel underneath this button as the button and hammer return to rest, the jack would now rest under the button instead of the higher hammer butt leather. In this elevated position, the hammer would be poised for another strike from a resting position that is closer to the strings, without requiring the piano key to return fully to the top of its travel. So, fast repetition is enabled.
That's my amateur guess!