It helps to know the history of the C&R designation. When GCA '68 was being drafted, there was a lot of outcry from the collector community, many of whose members are wealthy and politically powerful. They didn't like the idea of having to go through FFL dealers for interstate shipping of high ticket collectors' items. So they prevailed on Congress to allow them to have a collectors FFL which would allow most of the shipping privileges of a dealer, but only in items which the (then) ATTD would specify. After the law was passed, ATTD ruled that there would be no blanket designation of C&R, and that anyone wanting some specific guns so-designated had to apply by letter and the decision would be made on a case-by-case basis.
That got to be so burdensome for the (now redesignated) BATFE that they decided to issue a blanket definition of 50 years old to cover almost all the items collectors covet. The exception, on request, of a specific gun or specific guns by serial number or characterization still exists.
An NFA firearm can also be a C&R, important to note because in an interstate sale of an NFA item that is also a C&R (say a WWII Thompson SMG), the item can be shipped directly to the buyer if he is a holder of a C&R license; no "Class 3" dealer is required on either end.