what kind of tool do I need for this?

["Evangelos G. Makris" <emakris@hol.gr>]

What is the kind of tool I need to remove the 
differential oil drain/refill taps on my 66 Sprite?

They are square, almost 0.5", but I am not so sure.

It looks like a 0.5" socket extension with the 
little "ball" removed could do the trick. Wish I
knew how to ... remove the ball.

Local car hardware stores only stock hexagon (Allen)
keys for this operation, metric, the 11mm of which is 
too small and the 13mm won't fit.

Evangelos