This a keg returner. See, if you've got an empty old keg, or even a part-full one, you could fill it up with slops and then re-pressurise it. After that, I suppose you could sell it. Perhaps you could even sell watered beer. But who'd do such a thing?
It's just a big funnel screwed onto a slightly modified coupler. I'm sure there are legitimate uses for such an item. I just can't figure out what they might be.
Of course, similar devices are available for filtering back into cask.
This is Jon's personal "blog" - I work at an independent microbrewery (a small-scale, artisanal producer of “real ale” and other beery treats), based in the Furness area in Cumbria (or N. Lancs if you'd rather). Or a "Craft Brewer", if you like. We're known as "Stringers", or "Stringers Beer". I don't just make beer - I also sound-off in half-informed rants on a variety of subjects. Like here.