Just got an email back from the ebay ad you sent. The seller said they have two original 4198s carburetors but one has a different sized idle mixture screw than the other. They kindly offered to sell just the base plate from either carb separately. I'm thinking someone has replaced the base plate on their carbs at one point or another. Assuming I get pictures of the base plate and the passages on it match up with mine, are the base plates interchangeable on a carter BBS?