I've serviced a mats wheels which use DDG hubs that look very similar. However, if memory serves, there are two different designs. One has a nut (12mm) that unscrews from the non drive side and the freehub drops off. The other, you unscrew the freehub itself from the non drive side and it drops off. However, one means turning clockwise and the other anti-clockwise. Try it one way with a bt of force, then the other wide the same force, then the other way with a little more force, then back with a little more force. Build up like this until you feel something move!
Did it really take me 19 minutes to get around to typing that reply! Anyway, lots of inhouse hubs have similar looking freehubs. The DDG looks similar as does some specialized hubs. Maybe post a pic on here, particularly the threaded connection and see if anyone has seen similar.