Auschwitz Tattoo Equipment Found in Poland

this is a fascinating historical find - the metal stamps used for tattooing the Jewish inmates at Auschwitz has recently been found in Poland.
According to YnetNews:

Metal stamps with embedded needles used by Nazi soldiers to tattoo Jewish inmates at the Auschwitz death camp have been discovered in Poland, British newspaper the Daily Telegraph reported.SS soldiers used the small stamps, consisting of a two, two threes and a six or a nine, to tattoo inmates as they were processed on their arrival at the camp in German-occupied Poland, the paper said.According to the report, Auschwitz museum director Piotr Cywinski said the discovery was "one of the most important finds in years." 
"We never believed that we would get the original tools for tattooing prisoners after such a long time," he told the Telegraph. "The sight of a tattoo is getting rarer every day as former prisoners pass away, but these stamps still speak of the dramatic history that took place here even after all these decades."
The metal stamps were slid into a wooden block to form a number, which was pushed into the prisoner's flesh. Ink rubbed into the wound created a tattoo.
Some of the prisoners got the tattoo on the chest, the report said, but most were tattooed on their arms, and the numbers became a hallmark of Auschwitz's inhumanity.
"The stamps will become a valuable exhibit in forthcoming exhibitions," Cywinski noted.
Auschwitz was the only Nazi camp where prisoners were tattooed. At the initial period of the death camp, identification numbers were sewn onto inmates' clothing, but the procedure was changed after the clothes often disintegrated and due to the guards' difficulty to identify the prisoners who had been stripped before death.
The exact location of the discovery is unknown, the paper reported, citing the Auschwitz museum as saying that the finder of the stamps wishes to remain anonymous.

I got goose bumps when I first saw this article...  though it reminded me of a question I always had - why only by Auschwitz did they tattoo the inmates for identification and not at all the concentration camps?
I did not know that the tattoos only contained twos, threes, sixes and nines.. why was that?
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