very cool.
I do not have that specific winder in my collection, but I would suspect that the part you asked about is the "full bobbin" disconnection bit. Basically once the bobbin is full it moves that out of the way and stops loading thread. However, IF it fits between the ends of the long bobbin it COULD go behind and be a device to provide pressure and tension to the winding process. I'd have to get my hands on it and play to see what it does.
I would clean the tension disks for the bobbin winder as much as possible and try again.
FYI - MOST of the early long bobbin machines did not have thread arms to spread out the windings. It really was done by guiding it my hand most of the time. This is also a reason that there was a market for disposable wooden long bobbins that came pre-loaded with thread.