Stefan
That looks like it might be a 39 to 25mm reducing adapter.
Durst had those adapters to use the 25mm Schneider thread lenses in the 39mm LAPLA lens panels. The adapter avoided them having to make various 25mm lens panels. They make 39mm lens panels and those that want to use the 25mm lens just get the reducing adapter. It should unscrew from the LAPLA. HOWEVER, the question is does it unscrew up or down. From the pix, it is hard to tell if there is a flange on the top of the adapter. If there is a flange, then you would unscrew it UP (towards the camera in the pix). The trick is to use a tool to push against the small ridges and not damage the ridges. Unless you don't need the adapter, then damaging it won't matter.