Robbie's picture shows the brake drum before the fracture occurs:

 [ Music roll brake drum in Ampico B drawer
 [ http://www.mmdigest.com/Attachments/16/03/06/160306_205130_Music_Roll_Brake_Drum.jpg
 

Here are two drums after fracture:

 [ Ampico B drag brake drum, brass split due to pot metal expansion
 [ http://www.mmdigest.com/Attachments/16/03/07/160307_181442_IMG_5206.jpeg
 

It is just a matter of time, but original brake drum brass rings will
fracture once the pot metal swells enough.  The brass ring surrounding
the pot metal will merely stay stationary during roll play as the pot
metal rotates within.  Thus, all braking action is lost.

When replacing the original brake drum, the replacement must not be
solid -- it must be hollowed out as in the picture.  This allows the
drum to overlap a part of the transmission frame.  If it is not hollowed
out, the tracker mechanism will be restricted to about 50% of its
normal movement.  Tracking will be inadequate.

As Don Teach states, this drum should be brass to properly function
with the Prony brake webbing.

Bob Taylor
Missouri