Hello again, let me post some pictures of the Parker 51 with side hole in the barrel and a number 3 in the Clip (rear), I received some answers that I appreciate but do not get properly locate this model parker 51, I have two pens Parker 51 Aerometric with the number 3 on the back Clip- maybe someone can give me some information about- What does this number and the hole in the side of the barrel ?. Thanked advance