That’s got to be a 3rd-party electrical box, B&O probably made only the foam inserts themselves 😁 I am not sure how many of these were really sold (I don’t think any of the 90’s B&O speakers were in any way water-proof?).
Not very practical either to be disconnecting the outdoor units if the temperature falls below 0° (as the instructions say!).
I like how they call the older rectangular MCL transceiver the “oblong” one 😀
Neat find anyway, I like these obscure accessories!