I don't think the Penn Central bought any new locomotives in its last couple of years of independent (pre-Conrail) operation. The final deliveries of GP-38 (highest numbered unit, which I suppose is the last, 8162) and U23B (series ending 2776) were both, apparently, in October 1973. (Internet source for 8162, McDonough's (sp?) "The U-Boats" for 2776.)