I have a Durst Lapla 39 that is meant (obviously) for 39mm thread lenses.

Please have a look at the picture.

2013-04-15 21.25.09.jpg

It does seem to have a center part, that looks to me like a thread reducer that might be removable in order to use lenses with a larger diameter. Is that correct ? I did not succeed in removing it. Is there a trick ? Is it a left or right-handed thread ? Or is it not removable at all ?