Ah sorry, newbie mistake.
The bike is a late 80s Bianchi Campione D'italia. The rear derailleur is Ofmega masters and I believe the downtube shifters are Ofmega masters as well. It certainly looks like the pictures on velo-base. (Sorry I haven't posted enough to link urls)
With the crack on the top plate, you're saying that it's possible the pack is shifting every time I move the lever? I'm not sure about the material of the top plate, it's all black, I'd say it feels like plastic? There is another metal washer wedged between the screw and the top brake.
I'll give the local bike coop suggestion a shot.
Thanks Andy.