The LokSound by ESU 21MTC Adaptor Board is for adding a 21MTC interface by hard-wiring into a non-DCC ready locomotive. With its compact dimensions of 27mm x 17mm it is almost the same size as the decoder and is also equipped with an extra mounting lug that can be easily broken off if not required.
 
The board has 21 annular rings making clean wiring of your loco possible. Simply hard wire in the board and you can plug a decoder in when you're ready. This adapter board enables you to use all additional functions of the 21MTC connector (such as speaker outputs).