Most of the nipples with the external hex are 1/2 in. internally and don't have the internal 3/8 sq. drive. I prefer them myself due to less restriction. As for getting them on and off, I have found that most sockets have a bit of a chamfer to them. This makes them slightly easier to slide onto a bolt head or nut, but creates a problem when dealing with thin hexs like yours. What I have found effective is to sacrifice a six point socket by grinding the opening down to the point where the chamfer no longer exists. This way you can usually get it to grip the hex. If all else fails an "easy out" or screw extracter should work. If you have to resort to this I would try to polish the scars out of the interior of the nipple before reusing it.