Note: Since originally posted, there was a missing connection for pole [M-] position [D] to signal ground. It is shown in the schematic at the top. It was alluded to in the wiring diagram by the mysterious "X" at position [D] on the [B+] pole.
The winner of this lottery was andywelsh on 3Apr2008. His prize is only the certainty of really knowing that he really knows.
I have found an easter egg with this design. I changed my B+M+N setting to M only, however:
On my superswitch there are 'in between' positions, similar to the stock fender switch, where two sets of contacts are switched on. These give B+M+N in positions 2½ and 3½ in stock mode, and (I think) B*(M+N) and N*(B+M) in series mode. It's fiddly to set the switch there but it could be useful in a studio situation.