The setup instructions are a bit opaque. This is what the online HD621 manual states:

SmartLink Output - These four phono sockets each carry a pair of digital audio channel signals at up to 96 kHz sample rate: 1/2, 3/4, 5/6 and 7/8. These are encrypted to Meridian High Resolution (MHR) specifications when carrying high resolution data.
Detail of SmartLink output section.

There is no indication which channel is associated with each cable. The thread mentioned did not correspond to my config as it had D1/D2/D2 hooked to inputs that are associated with MD1 AND MD2 without fully filling either.

The OA13 outputs are listed in my config as OA13 #1 card Front/Centre/Sub with the connections as
X1/X2 into my 5 channel amp A1-2 which I assume is L/R. X3/X4 should be center and sub while my second AO13 shows X3/X4 for my surround speakers and I assume this is L/R also.