Middle Pedal on 1899 George Steck Grand Piano

I have not seen this type of function on a grand piano before. It seems to be moving the damper upstop rail. I don't understand what the advantage of pivoting the upstop rail forward would be. I'm looking for answers!

[@hayforker]

I think that’s not the upstop rail, it’s a sostenuto mechanism.

[@hayforker]

… or it could be serving both purposes.