The cube, according to specialists at the JRC’s Institute for Transuranium Elements (ITU), was produced in 1943 for the Nazi nuclear programme and was used in the lab of famous boffin Werner Heisenberg (of uncertainty principle fame). The plate was apparently part of experiments by Heisenberg’s collaborator Karl Wirtz.
Most historical analysis, with hindsight, suggests that the Nazi nuclear research programme never got very close to developing an atomic weapon. There was no equivalent of the Manhattan Project; rather, different lines of research were followed in different labs. Furthermore the Germans were hampered by having driven many top physicists out of the country with their anti-Semitic policies, and also by drafting other boffins into the army to fight as ordinary soldiers.