Up until the line gained its independence from Penn Central it's lettering practices pretty much followed that which had been dictated to it by parent New York Central (not changed when NYC became a part of PC) in 1976. The link below will take you to the P&LE hopper car page at RR Picture Archives which shows 70T hoppers in the 61000 - 64000 series with the NYC patterned lettering.
In '76 when the line gained its independence it took a different course in lettering equipment as a way to help convey that it was its own company once again after nearly 100 years of control by the NYC and PC. Among the changes was locomotives receiving large "P&LE" lettering, which was followed by new cars being acquired with the same large initials. It is difficult to determine by the number of photos that can be found on the internet exactly when the first cars were received with the large yellow lettering since photos are not very numerous on the internet but it would have been after 1976.
