looks like a pb1 to me... or maybe a ver R

definately not a ver S

I have previously mentioned this but anyway.
At the bottom of my work in progress for the superfour http://litetek.co/Carb_Kit_Honda_CB400_NC31.html you will find a complete factory parts book that cover all models from the FIIN to the FIIIV. (right click, save as, etc etc) On pages 14 and 15 are the frame and engine number tables for the relevant model.
cheers
Blair