I just purchased one off Craigs after a careful search. Closer examination showed me there is no height adjustment on this one, sits about 21" before compression, which for me is mighty low. The Ludwig screw/knob is there for apparently no reason other than cosmetics? I don't see a function. I see on eBay both adjustable and non-adjustable Porto-Seats looking at the holes, or absence thereof in upright rod pictures. Am I missing something about adjustability or is it simply that Ludwig made both? I would suppose the going market rate for adjustable is higher? Any knowledge would be appreciated.