The original type was 2001 acording to the system manual for 1988/89 which I have in a original printed copy.
It looks like the old MCP type tranciver panel but has only a send and recieve circuit installed.
It should be connected via the SENSOR plug on the back of the BM5500.
It looks like one used MCP tranciver panel which I got. Grey with just the word sensor to the left and Bang & Olufsen printed to the right. The plug has 6 pins and the cable is grey.
I might sell it but it came with the very rare Beolink 1000 remote/code converter kit for BM6000/8000 which I still need a special cable for to be complete.