That is the business of archives, to know what is in their holdings to have them catalogued and to respond to such requests. After all, if the IPN had sourced their "human soap" from the Hague archives they must have gone through a similar process.
I did give them the evidence reference number it was submitted as by the Soviets, USSR-393, to help them. There was really anything else I could be expected to provide.
As a rule, most exhibits were returned to the authorities who provided them in the first place. So that, for example, the alleged flayed human skin that was displayed as from Buchenwald ended up, IIRC, in the Smithsonian.