O.S. "F" or the YS 4 stroke plug. They're the same.
Grab your ankles and look here.
http://www3.towerhobbies.com/cgi-bin...&I=LXCB28&P=ML

Then look here
http://www.centralhobbies.com/glow/glowplug.html

Now you know how Tower can afford to give you those Super Saver deals.
Paul