...extraordinary sets with switches etc. For example:

Intercommunication Sets, Multiline Sets, Intertrough Switches, P.M.B.X.-Sets, Small Switchboards

Actually I'm looking for Schematic diagrams and other technical informations about the following Ericsson Private Manual Branch Exchanges::

P.M.B.X. Ericsson
or similar Sets
ADD 1311, ADD 1321 and ADD 1331

P.M.B.X. Ericsson

Ericsson (PTT Netherlands)