Many knowledgable car people may not even be familiar with the term NOS, which stands for New Old Stock. This term is given to parts that are auto parts from the original (automobile/motorcycle/tractor/ etc) manufacturer for repair or reconditioning on their originally sold product. Sounds confusing, but really is not.
You see AC/Delco, AC, Rochester, Delco-moraine, and several other manufacturers were all part of the original GM network (owned by GM), and all of these companies supplied back to the parent company(s) (Chevrolet, Olds, Pontiac, Olds, GMC) for parts required to make the new cars being delivered to dealers. These companies also were responsible for supplying parts (at a profit) to the companies dealerships to repair the new cars, should they be involved in an accident, warranty, or in need of a repair by aftermarket shops. The old rule of thumb for manufacturers were to be able to stock parts for vehicles for between 10-15 years past their initial production (purists might say it was more, but work with me in this case), so AC/Delco needed to be able to supply a 1970 GTO ignition switch to a dealership that required one as recently as 1985. If the dealer bought one of these ignition switches (in 1970), but never used it, and then tried to return the part to GM in 1986, GM was under no obligation to take the return from the dealer, as the time had elapsed for GM's "responsibility" to produce parts for the continued operation of the new 1970 GTO.. Thus a "new old stock" part is at a dealer with no use for it. These parts were sold by the dealer to private citizens to help lower their inventory costs, and they would rather get something from the part rather than throwing the obsolete product out. (A new industry is born with the "reseller" of NOS parts, see e-bay motors for listings from people who buy out entire dealerships)
Nothing fits like an original part, and although more expensive, NOS parts are superior to reproduction parts in many cases. If buying an NOS part, you need to see if it is in the original wrapper, as many of these parts are now reproduced, and sold as NOS on e-bay with the disclaimer NOS-NEW, this means that somebody has duplicated the old part, some times with inferior methods or materials, and are claiming the part to be NOS. Many of these parts are of decent quality, but if they are reproduced by 40 year old dies or molds, the molds may not be as "crisp" as they were when manufactured new, as these dies or molds age with use, like everything else, and you may wind up with a body panel that does not have a crisp line to it, or an emblem that does not have well-defined edges. another example would be an emblem that is now applied to a car with an adhesive backing, VS the original pin type of attachment.
If a person is doing a nice cruiser for their own personal enjoyment, it might be an intelligent decision to buy some reproduction pieces to keep the cost low, but if you are truly trying to return your car to it's as delivered state, and/or have a rare car, it is prudent to depend on the NOS parts to restore these cars. The low cost (reproduction) emblem, might be the first thing to fail on that gorgeous frame off restoration, and it can be very frustrating chasing these little details on a car you just payed an enormous amount of money to restore.
OEM Manufacturer (FORD, GM, CHRYSLER etc)- Original car/part manufacturer
NOS -New Old Stock . Part made by OEM manufacturer, licensee, or subsidery.
NOS-NEW- A new part made to replace discontinued parts, in many cases under license from the original manufacturer. Many times these are of higher quality than the "repopped" parts. They can be exceptional (See Goodmark body panels, Gardner Exhaust etc), but many false starts from early imported products have tainted the reputation of many products.
Reproduction (Repop)- Any part not made by or licensed by the original manufacturer. Various manufacturers of widely varying degrees of skill and quality. These can be accurate (good-no problems) to (junk) ill fitting inferior panels that take longer to correct, than trying to find an NOS panel. You may be buying a fender for your mustang from a manufacturer that is paid by the piece to produce product. (No quality assurance). Keep in mind that these products in many cases are not packaged correctly to assure a damage free ride as well.
I do not pretend to know all the answers, but hopefully I have waded through some questions, as I get this question regularly when I list parts for sale. I am not necessarily against reproduction products, but I want people to realize that sometimes you may have "unfulfilled" expectations, on those products, as compared to the original NOS product. Hope it helps........Brian