It is a "Transient Voltage Suppressor" (TVS) and I was was told this is a zener diode (this could be wrong). Yes, it seems odd that Pertronix does not build this into their ignition and does not sell the part (and instructions on how to install it).
The specifications for the four part numbers are at the bottom of this link (they call it a ZENER TVS):
5KP20C
5KP20CA
5KP22C
5KP22CA
http://www.sensitron.com/comm/documents/datasheets/1249.pdf Tim
