Very few hubs allow a fixed wheel lock ring on both sides. Without the extra left hand thread for a lock ring (and the lockring itself) the fixed sprocket will screw on in place of the freewheel, but unscrew again when you apply back pressure.
You might be lucky, when you remove the freewheel, to see the extra threads.
Very few hubs allow a fixed wheel lock ring on both sides. Without the extra left hand thread for a lock ring (and the lockring itself) the fixed sprocket will screw on in place of the freewheel, but unscrew again when you apply back pressure.
You might be lucky, when you remove the freewheel, to see the extra threads.