The Nimbus software security solution...
Dongles were all the rage in the 80s & 90s for securing access to licenced software and some hardware.
A specimen of a nimbus dongle has illuded the museum (assuming any were ever released to the public), so information on its robustness in protecting software is limited.
RM says the main reason for the inclusion of the dongle is to keep software licening costs down when the software is held on a network server. This placates the software houses that were demanding multiple-user licences when used on a network. The dongle ensures that it can only be used on the machine with the inserted dongle (source).
The dongle probably contained a single NMC9306 EPROM of the type used by the Nimbus to store configuration data.
This is suggested by looking at the circuit diagram which shows the softkey sharing the same data lines.
The June 1986 sales catalogue states, "The Softkey contains a firmware code unique to the user. The software contains the same code and needs to be matched with the softkey before it will run"
This in itself would probably have been sufficient due to the uniqueness of the dongle being a sufficient obstacle to duplication.
Based upon the connections from the BUS it appears reading as well as writing could be possible to the softkey.
The connector is an 8 pin Edge connector: