I'm not sure what you mean by "Clamshell Adapter". Typically that refers to a rear hub adapter to mount the sprocket to the hub as opposed to the "rag joint" or rubber donut style of sprocket mount. I don't believe dax sells clamshell adapters because the machining is very size critical and every hub is different. The clamshell adapters I've seen are all made and sold by machine shops. Here's an example:
SPROCKET ADAPTER Can you be more precise?
Tom