This adapter board consists of 2 parts. the lower half plugs into a 2nd SID socket and provides the usual chip select and audio output pins to connect to the uSID64.
The upper half (Audio Board) can be detached from the lower half and is optional. The whole board can be used in the new uHeld64 from v1.5. The upper 3 pins AU2, 9V, GND are connected to the upper keyboard board and the middle pins A5-A9 to the lower mainboard board. This way you get a stereo SID with the uHeld64 in combination with a uSID64 and the 2ND SID adapter board.
If the A5-A9 pins are connected, the 2nd SID of the uSID64 will automatically respond to the SID addresses D420, D440, D460, D500, D600, D700 without the need to constantly change jumpers for a specific SID address.