Is this LEGO an authentic original? I found this LEGO piece in a second-hand LEGO stock. I have never seen a brick like this, and I can't even find the LEGO write on it.

Is this LEGO brick original, maybe vintage or some kind of special edition or is it just fake?
This is a Minitalia brick - Lego produced different bricks for a while in the seventies for Italy (per Gary Istok's latest information: because the original LEGO was deemed too expensive). Minitalia bricks have weaker plastic (which shows on the studs on your brick) and don't use the Tube system underneath the brick, but an older patent which has the X marks. The gripping power of the X marks is less good, but you will find that the X marks actually allow for half-stud offsets, which you cannot do with the tube bricks :). All this is detailed in Gary's wonderful book/DVD