To be advertised as NOS it must truely be "New Old Stock" ie stock from the 70's that is brand new but has been languishing in the corner of a warehouse for the last 30 years.
A lot of people use NOS on eBay and lie, but as a reputable dealer, DSS would not do this (apart from the fact that it is illegal in the UK to misdescribe goods for sale)
Reproduction seats are new manufacture and are cheaper generally due to modern manufacturing costs. There is a strictly limited supply of NOS! Repro seats may also be made from better (more modern) materials.