Unlike “castings” or “blanks” which are formed as a single piece so that a fire-control cavity has not been made, EP Arms uses the biscuit specifically to create that fire-control cavity during the injection molding process. As described in your letter, it appears that the sole purpose of the “biscuit” is to differential the fire-control area from the rest of the receiver and thus facilitate the process of making the receiver into a functional firearm. ATF has long held that “indexing” of the fire-control area is sufficient to require classification as a firearm receiver. Based upon the EP Arms manufacturing process, it is clear that the “biscuit” serves to index the entire fire-control cavity from the rest of the firearm so that it may be easily identified and removed to create a functional firearm.
Keep in mind that courts are generally highly deferential to agency determinations, so I wouldn’t give this much of a chance in court. But it’s noteworthy that ATF has “long held” that indexing constitutes creating a receiver. Where in the Federal Register can that be found? Can’t find anything in the code of federal regulations either. It’s probably found in other determination letters. This isn’t rule of law, it’s rule by bureaucratic whim.