I'm sure the pan in the CB kit is pretty inexpensive by looking at it. It is just a kitchen bowl that fits. This is what Harry Soo said about the pans:
"If you are using a full overflowing load of briquettes for a 12+ hour cook, the standard deep WSM water pan won’t work because it sits too low and will bump up against the top of your briquettes. No worries. Just remove the deep water pan and use the silver aluminum heat shield instead. You need to unsnap the heat shield and wrap it in double layer of aluminum foil and use that ultra-light pan in place of the deep water pan. If you have access to a WSM circa 2008 and earlier, those older WSMs come with a shallow water pan that does not bump into the briquettes."
Plus they have different part numbers. Don't think he was talking about 18.5" but he could have been.