7 years for sure, 10 likely correct in some instances.
When I got my 71 Cuda in 1976 I used to be at my dealer all the time checking on the availablity and ordering parts, stuff like wheellip mldgs, chrome headlight bezels, rocker trim, fender louvers, valances, just better stuff than what was on the car.
They got so tired of looking stuff up to order for me, that they gave me a ratty 70-71 parts book and told me to bring the numbers in I wanted.
In or around 1980 I went in with the correct part number and had my dealer order a 71 header panel after putting a ding in mine.
Parts guy told me it was NLS... No Longer Serviced( same as NS-1 in the US ).
They still priced out and I kept bugging him why, so , to brush me off he gave me the phone number of the Mississauga Depot, said to call and ask to talk to the 'parts specifier'and ask him, which I did.
Specifier said " Look there are about 75 of these in the corresponding bin but I cant sell you one, I'd give you one if I could but I cant even do that.
Once they are deemed a 'slow moving part' they are taken off the system, non-servicable item, and must be scrapped"
Moreless to say that, in 1978 or so, people weren't ordering a 71 Barracuda header panel every day, so they had to make room in that bin for more current parts.