Hi guys what do NOS stand for?

new old stock
even though its a part from an old car , it is still brand new

Correct but slightly more accurately it's a part that is unused but has been laying around since the vehicle was in circulation.
In the case of a rubber seal or something perishable it might as well be a POS as it could have hardened over time and be useless.

One would normally expect a NOS part to be genuine and not a reproduction item (as these are not normally produced until some time after a vehicle is in production).

well correct, but I'd imagine the car manufacturers would order extra quantities of the parts bin's stock then needed for production line fitment, intended for spare parts supply. Don;t forget that there are alot of outside companies making bits and pieces like say heater ducts or door mouldings and not all of these companies keep making these items or go out of business etc. that's why (apart from lack of demand) thats why alot of the parts are NLA (No Longer Available, so no one has to ask that one either ;) hopefully rare spares will see the light and start making the hard to find jap parts soon.. crazy if they don't really.