Help understanding a Darlot Portrait sliding aperture

[Lethargus]

I finally picked up a Darlot Cascet set and have been pleased with some of the shots I've taken (mostly of my wife who has not approved their posting to the internet yet.) What I'm not understanding is why the waterhouse slot can move. It's part of a cascet, so I thought it might be for different lenses to keep it centered, but all the lenses appear to be the same distance apart in any configuration. I took test shots with it fully forward and fully back in both wide open and fully necked down. Between the four shots I see no difference aside from what you would expect in reduced aperture. No difference between the forward or back positions. Does anyone have insight into the purpose of the movement? And what the threaded knob is for? I thought it would lock the waterhouse slot, but it appears to have no affect at all.