Corrected entry: In the internment camp scene, many Jews are stripped naked. However, many of the men are not circumcised.
Question: Was the one-armed man a real person?
Answer: No, the one-armed man is listed as "Mr. Löwenstein" and played by Polish actor Henryk Bista. He is a fictional character.
Answer: According to an internet source, the one-armed man, Itzhak Stern, was real. Stern was a Polish Jew who worked for Oskar Schindler as an accountant and assisted in his rescue activities during the Holocaust. After the war, Stern moved to Israel.
Stern and the one-armed man are not the same person. The one-armed man, hired by Stern himself, dies during the movie, and Stern, as you wrote, survives.
Factual error: In the beginning, when the Germans are setting up the tables to record the names, one German puts down a plastic stamp pad. Stamp pads of that era were metal.
Suggested correction: Not true. Rubber stamp pads were invented in 1866. By WW2 they were easily available.
I do not believe the mistake refers to the stamp itself or the ink pad, but to the container holding the ink pad. The stamp is made of rubber, but the ink pad should be contained in metal.
Personally I think it is a metal stamp pad. Maybe a second pair of eyes to confirm? At 1:31 https://www.youtube.com/watch?v=9UoF6uIQOK8.
It could have easily been celluloid or Bakelite - both had been around for decades.
Correction: Many of the men would not be circumcised because a lot of their families were not practising Jews and were only deemed to be Jewish by the Nazis even if only one grandparent was Jewish. In addition, there weren't only Jews in the camps, but Gypsies, POWs, homosexuals, political dissidents, and other "undesirables."
Yet it is true that not only Jews were in camps; only they used the David star in their clothes, and in the scene, it is noticeable it is stitched to the clothes they were carrying. So the scene intended to depict Jewish people. Regarding the other real historical reasons presented to explain why being Jewish, they were not circumcised, I have no idea what percentage of them would apply and what the chance of it would be in a randomly chosen scene. Or even if that was intentionally considered in the movie.