I thought pistols were usually the frame. Why is the mkiii different?The part of the firearm that has the serial number is the actual firearm. That is the part that must be transferred to an FFL.
On a Ranch Rifle or Mini-14, that is the receiver. It doesn't really have a lower.
On a MKIII .22 pistol it's the barrel/receiver assembly, so buying a lower would not involve an FFL. But if you want to buy a new barrel for it, it must be done through an FFL.