Unusual resistor symbol: resitor with Z overlaid
Doing a google search of "Resistor Symbol" and then looking at images, I came across this.

http://www.electroniq.net/electronic-tutorials/resistor-types-symbols-parameters.html


  


According to the site, its a varistor tolerated symbol.


  a: resistor , general symbol
  
  b: resistor , tolerated symbol
  
  c: resistor , un-standard  symbol
  
  d: resistor with variable resistance
  
  e: resistor with mobile contact
  
  f: resistor with mobile contact and  stop position
  
  g: potentiometer with mobile contact
  
  h: potentiometer with mobile contact, general symbol
  
  i: potentiometer with preset adjustment
  
  j: two plugs resistor
  
  k: shunt resistor
  
  l: thermal resistor
  
  m: thermistor general symbol
  
  n: thermistor tolerated symbol
  
  o: varistor general symbol
  
  p: varistor tolerated symbol