That makes it impossible to retro fit FA parts, assuming a person could find or make them. A person would have to mill out the internal receiver to have space to fit the parts and when that is attempted the mill would break through the receiver and render it useless! The ease of conversion to FA is a consideration with all new imports.
Other than "once a converted receiver - always a converted receiver" - wouldn't retro-milling existing semi-auto receivers with the same cut-out prevent the addition of FA part?
L