I wouldn't bet on you being right about this... I saw both NKP triple offset-side hoppers and various VGN hoppers on the Southern Railway into the mid-70s, over ten and 15 years after those lines were merged. N&W was never in a hurry to repaint their cars and I would bet if you looked hard enough (that is, if you wanted to include the earlier cars) that you could find shots of them well into your modeling era.
I was lineside in the 1960s and 1970s and I can hardly remember seeing any coal hoppers with 17" lettering. My collection of photos seems to support this.
Does anyone know when the H2s started to be built?
H2a hopper production:
1948 15500-17484, 70145-70159
1949 17500-19499
1950 19500-20336
1951 20337-22889
1952 22890-24560
1953 22561-25499
1956 26000-29999, 31000-31999
13,500 total
(from Andrew Dow's N&W Coal Car book)
Mark