As it turns out the flange is smooth at the threaded end, but as Dan suggests too long in either direction. The reason I thought it might work turning it around (as one poster suggested) is that I thought for some reason that the threaded end would permit the lens to go farther down into the flange than the opposite end. In examing the flange, though, it became clear that this was not the case as the non-threaded end is not only wider overall but has a wider opening. (In retrospect, this is clear even from the pictures I posted.) But thanks for the suggestion and I'm off to buy a retaining ring, giving B&H a piece of my mind for selling me something other than what was listed. Thanks.

Accidentally posted this as a new listing rather than a reply on the last. Hope the administratior feels free to delete this thread; didn't mean to add clutter. Sorry.