The GN and NP date from the 1960s, with the GN no later than 1967. GN started using blue in 67, and both the GN and NP became BN in 1970, so they have to be earlier than that.
According to one book I have, the GN changed from G ### to GNZ ### in January 1966, because the AAR standardized trailer reporting marks. So, the GN trailer has to predate that, meaning it goes back to no later than 1965.
