Well I do know I have not seen this before. So it wasn't anything laying on the shelves in 2006 or there abouts. We also know that Weber moved several times -Arlington Heights, Wood Dale, Palatine, Huntley. I'm not sure where R&D moved to these days, I don't keep tabs on them... so a lot was shuffled around in all those moves.
They did have Factory Closeout sales each year in the summer up until 2004/5. They discontinued them in 2006 from what I was told -they didn't have one in 06 when I started. They would sell off unique items, limited edition stuff, open box -so you may have that cast piece without the rest of the parts.
We actually have 2 things here. One is the unique bowl with the vent on the bottom, the same ones I used on the 22. Someone who is at home (as I am at work right now) should be able to judge that diameter with a 14 and 18 they have laying around to determine the bowl size. Because they are awfully close to the leg couplings, right? If it had stubbys or full length triangle, good question -which again, was it a 14 or 18.
But given the large cast piece, an 18? I suspect those ridges are on the underside and rest on the edge of the bowl. Whether or not there are other accoutrements, good question.
I have a source I can reach out to and i'll let you know what they say. Cool find!