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.
That is a very tough call. The pad sounds plastic when placed upon the table as the sound is rather light whereas a metal pad would more likely have more of a thud than is heard.
It could have easily been celluloid or Bakelite - both had been around for decades.