Hi, that SOMETHING is the BeoLink Converter NL/ML aka BLC:
The BeoLink Converter NL/ML is a converter between the Network Link and the Master Link system and makes it possible to integrate Network Link products and Master Link products. Search for BLC on the 2012-2022 archived forum and you will find a lot of info. You will need to find one on the second hand market. Here in the Netherlands they are sold for 200-300 euro. Good luck.
The BeoLink Converter NL/ML is designed to convert control signals and audio signals between Master Link products and Network Link products. Analog audio signals from products in the Master Link system are converted to digital signals by the BeoLink Converter NL/ML and transferred to the products connected on the Network Link. Digital audio signals from the products on the Network Link are converted to analog signals to products in the Master Link system.