He almost assuredly referred to Grade 4 as "4th class" and Grade 5 as "5th class" i.e., 5.0 to 5.9 climbing rating. You would see him say "middle 5th class" or "upper 5th class" in the ratings descriptions. The Appalachia article in 1960 is the best known of these publications--but the AMC kept notes throughout the period. Perhaps in the archives of the AMC?
Moreover, suggest that you find someone still alive who climbed in the Appalachian Mountain Club in the 1960s. I recall that they organized semi-annual trips into Millbrook throughout this period. All of these trips involved hiking in from the base, descending on the side of the cliff once the top was reached, and finding your knapsack on the talus slope at the base, then walking out. Even in the 60s the situation with landowners was "tenuous" and great caution had to be observed to maintain minimum profile when approaching Millbrook from the bottom.