quick dumb question, pinion nut

I asked Dr.diff about some parts and in an email back they asked for the "socket size of the pinion nut" to determine the pinion spline count. I would need to ask my father to check this for me, as I am overseas in Japan.

so what am I measuring, exactly?

while Im at it, the casting number is the last 3 of the serial, correct?

Thanks!