What is this device? (8+2 pin + string + label = "Test")
Something about it reminded me of a telex machine. Doing an image search led me to this:

Photo 1. Merten 465108 ADO Telephone Socket 8/8–Pin/White. Image source: Amazon.
The pin numbering sequence match your connector. I can't figure out if it's country specific but there was something similar used in Ireland. They faded out of use by the early 1990s with the rise of the fax machine.
So, I reckon its a telex circuit test plug.


Photo 2. A telex machine. Image source: sv.wikipedia.org.

ADo 8
(The Walzenstecker shown above was later succeeded by the more versatile 8-pin connector known as ADo 8.) This connector was used for a variety of equipment and had two 'coded' guide pins at the centre, to ensure that the plug is entered into the socket with the right side up, and to avoid the wrong plug being inserted into the wrong socket. The sockets are 'programmable'.


Figure 1. Pinout. Source: Crypto Museum.
TX a (a1)
TX b (b1)
RX a (a2)
RX b (b2)
Bridge to 6 1
Bridge to 5 1
-
-


The orientation of two small discs inside the socket can be altered in order to configure it for various applications. The image above shows the correct configuration for use with a teleprinter. Pins 1-4 are used for the same signals as on the earlier Walzenstecker. The bridge between pins 5 and 6 is optional 1 and is used on newer equipment to signal that the teleprinter is connected.

Source: Crypto Museum.