What do the letters W, Z and G stand for on an extension cord (USA)?
North American Standard   

W, White,  Neutral
Z , Black, Hot (120Vac/60Hz)
G,  Green, Earth Ground  

WJ-28-1
NEMA 1-15P  SPT-2(wire)     18-16/2C  AWG

Updated  P/N

thanks to david

WJ-23-1     5-15P  SJT/W   18-12/3C

E301276
JIANDE WANJIA ELECTRICAL APPLIANCE &amp; CABLE CO.,LTD.
INDUSTRIAL ZONE SOUTH OF BRIDGE, MEICHENG JIANDE
HANGZHOU, CHINA