Fixed threads
That is almost certainly what the threads on the NDS are, it's a multipurpose hub. I have the exact same arrangement on a 5sp wheel from a '61 claud butler. BITD you could ride geared bike to your fixed TT, turn the wheel around, compete, then ride home a again with gears. This is quite commonly seen on vintage hubs.
I walked into a famous east london HHSB DayGlo Fixie' shop a little while ago whislt using the fixed side, with the old 5sp freewheel still on the there, and the proprieter looked very confused. Bless