This is a total complete guess on my part, so please correct me if I am wrong, but I would suspect that the Rock Island 639, 641 and 642 were the last E7s to operate in regular service in the US. They lasted until the delivery of the RTA F40PH units, which was around 1977. I believe that all the former Penn Central E7s were retired earlier than the Rock Island units.
For a short time after they were delivered the RTA F40s pulled the "cattle train" with the Harriman coaches which had been the regular assignment of the E7s. Photos of the F40s operating with that equipment are very rare. The only preserved E7 I have seen is the PRR unit at the Railroad Museum of Pennsylvania.