I looked through AMERICAN CAR & FOUNDRY COMPANY 1899-1999 by Edward S. Kaminski, found a photo of ACFX 78689, lettered "Anglo-American Clays" built in July 1978, it appears to be the type of car the Atlas model is based on, It is described as a 13613 gallon, DOT 111-A-100-W1, non-coiled/insulated tank car with 100-ton trucks.
The earlist car pictured that resembles the Atlas car is CCLX 1674, describerd as 16834 gallon, DOT 11-A-100-W1, exterior coiled/insulated tank car with 100-ton trucks built in August 1975. It is lettered "Corn Products".
Note: the DOT numbers are the same although the descriptions are different.