The C&R isn't a state issued item, it is Federal. Some folks get confused between the C&R and the MD Designated Collector Status for Regulated firearms. They do two different things, but they both also help empty a bank account.
The C&R allows you to buy certain firearms, classified as C&R by the ATF directly, except if the gun is sold in the state of MD. Guns that are over 50 years old are usually (but not always) on the C&R list. Certain other new guns can be placed on the list if they fall into certain categories. Like unique or a commemorative edition.
Antique guns, guns made before 1898 are almost all unregulated or controlled.
The MD Collector Status exempts you from the one gun every 30 day rule and some other minor stuff.
Military stuff has to be prior to 1946, AND issued, that's the kicker, they did that on purpose so AKs will NEVER be on it...otherwise they must be SPECIFICALLY listed, on a case by case basis, based on "collectibility." (i.e., Dragunovs, Makarovs, Russian and Romanian SKSs that are under 50...) There are some FN marked Mausers from 1950's which are NOT CR, because not specifically issued...and they are NOT on the list...