They are tab washers, used to help prevent a fastening from working loose over time (and vibration). Theoretically if you torque a fastener correctly you shouldn't have an issue, but these provide a visual confirmation as well as a functional 'lock'on the fastener. This looks like a bogie back from overhaul recently, noted by the paint pen and fresh fastenings (and the very clean bogie!). The 377/6&7's have different bogies and as such different fasteners on the axle boxes. Its worth noting once a bogie is covered in brake dust its very easy for the whole bogie to blur together and not immediately notice the fastenings on various components.
Not all units will have this , but a few iI saw (pictured example a West Croydon- Victoria via Crystal Palace service) has lots of little silver things on the bogies, almost looks like a magnet inside has been hoovering up stray bolts.