Plessner’s estate is stored in the department of ‘Special collections’ of the Groningen University library.
A short version of the inventory sheet in Dutch can be found under the following link:
Inventory sheet (pdf-file)
The complete inventory file in Dutch can be found under the following link since the beginning of 2017:
Inventory file, provided by the Groningen University library (excel-file)
Furthermore, the library makes available the complete inventory file divided in inventory files of a) all correspondences, or b) all other archive contents. You can access them under the following links:
a) inventory file of correspondence (excel-file)
b) inventory file of other archive contents (excel-file)