despite the importance of the tattoos, as testament, symbol, and historical artifact, little scholarship has been devoted to the subject. There exist virtually no official period documents relating to the practice; what we know stems from anecdotal evidence contained in camp records and the accounts of those who were at the camps.
The Auschwitz Concentration Camp Complex (including Auschwitz 1, AuschwitzBirkenau, and Monowitz) was the only location in which prisoners were systematically tattooed during the Holocaust. Prior to tattooing, several means of identifying prisoners, both by number and by category, had been implemented; serial numbers were the main method. When they arrived at the camp, prisoners were issued serial numbers which were then sewn to their prison uniforms. These serial numbers were most often accompanied by different shapes, symbols or letters which identified the status, nationality, or religion of the prisoner. This practice continued even after tattooing was introduced.
Circumstantial evidence indicates that tattooing of prisoners was not systematically implemented in Auschwitz in 1941.
http://www.jewishvirtuallibrary.org/jsource/Holocaust/tattoos1.html
Thanks for the link.