If memory serves me, this is an old style Matsushita(Panasonic) Varistor or MOV.
The ratings I think are as follows:
ZnR= Zinc Oxide Resistor
20 = Size (Dia) in mm
S = Disc type with round leads
K = Tolerance 10%
221 = Maximum permissible ac operating voltage
If the Sxx has an underline, overline or both:
Sxx underlined = Type series Advanced E2
Sxx overlined = Type series Superior E3
Sxx both under/ overlined = Type series Superior E4
At the bottom there is usually a date code like 07 18, this would mean it was manufactured week 18 of 2007 for example.
Hopefully this will help you choose a replacement
rgds