This is my opinion and worth everything you paid for it.
To be C&R, a firearm must be 50 years of age OR for some reason be collectible and added to ATF's C&R list by name.
NOTE: For US made firearms, none of this matters. It can be C&R based on age alone. This applies only to imported military surplus firearms.
When we speak of the "original configuration" description, we are generally speaking of "military" firearms.
Remember that none of this existed prior to 1968. Before that date, guns could pretty much be imported, bought and sold at will. After that date, we were introduced to things such as 01-FFLs, C&R 03-FFLs and "sporting" firearms.
The 1968 GCA banned the importation of any former "military" firearm with the exception of those imported under the C&R clause.
(d) The Attorney General shall authorize a firearm or ammunition to be imported or brought into the United States or any possession thereof if the firearm or ammunition—
<snip>
(3) is of a type that does not fall within the definition of a firearm as defined in section 5845(a) of the Internal Revenue Code of 1954 and is generally recognized as particularly suitable for or readily adaptable to sporting purposes, excluding surplus military firearms,except in any case where the Attorney General has not authorized the importation of the firearm pursuant to this paragraph, it shall be unlawful to import any frame, receiver, or barrel of such firearm which would be prohibited if assembled; or
<snip>
(e) Notwithstanding any other provision of this title, the Attorney General shall authorize the importation of any licensed importer, by the following: (1) All rifles and shotguns listed as curios or relics by the Attorney General pursuant to section 921(a)(13), and
This is were the "original configuration" comes into play. Any military firearm that was imported after 1968 had to be on the C&R list as a collectible firearm. It is only considered a C&R if it remains in its original configuration. (Remember, JFK was killed with a imported military rifle that was not in its military configuration.)
If you change the configuration, you remove it from the C&R list and it is no longer a legal import. A prime example is the current crop of Yugo SKS rifles. They are legal in their military configuration. If you slap a folding stock, pistol grip and 30-round magazine onto it, it becomes an illegal rifle for two reasons. First, it isn't a C&R any more and cannot be imported because it is "military surplus" and second because it violates 922(r) which prohibits the assembly from imported parts any firearm that it not legal for imporation in its assembled form.
If the Enfield was imported prior to 1968, it is a C&R based on age alone, regardless of configuration.