Jon's second post gives the idea! In restoring or building a classic today to Day-2 spec's it means; using the parts, customization and paint themes that were done to these cars back in the day.
An example would be a COPO or any classic car bought new, stripped and set-up for the drag strip and raced now 40+ years later it could be restored as "new" (factory stock) or "Day-2" so it is like the car was initially customized back in teh day.
This goes for show cars from the day as well as street cars too. Another example would be my '72 Nova, it has old school restored American Racing wheels, the engine has M/T valve covers, white painted headers, Hi-Jacker shocks, 70's style upolstered seats, a Sun tach and Stuart Warner guages and a bee's eye air cleaner so far. Future mods will be a restored original torquer manifold and a 70's Pionner Super Tuner and 6x9 speakers. It's 2012 but being built like it was 1974...