PYB Nano Compatible With Pyboard [EC-DTA00421N]

I bought one of these nano compatible pyboards, but now I want to reflash the firmware on the board. The instructions say to do that the 3.3v pin has to be connected to the DFU pin, but there is no DFU pin on the board. There is one marked BTO, and I'm wondering if that's the DFU pin. If not which pin has to be connected to 3.3v to put it into DFU mode? 

FWIW I tried connecting 3.3v to the BTO pin (they are right next to each other, as on the original Pyboard) but dfu-util did not detect the board. So maybe it's another pin?

Thanks, Ian