Plessner’s estate is held in the department of “Special collections” of the Groningen University library.
A short version of the inventory sheet in Dutch can be found here:
Inventory sheet (pdf-file)
The complete inventory file in Dutch can be found here:
Inventory file, provided by the Groningen University library (excel-file)
The library also makes available the complete inventory file divided into a) correspondence and b) all other documents. They can be accessed here:
a) inventory file of correspondence (excel-file)
b) inventory file of all other documents (excel-file)