The Marshall "copy" of the Bassman. Celestions, KT-66 tubes (later EL-34's), different transformers. What a sound! The 4x12 that ultimately came after the 8x12 is so much of the sound too. Don't know wether those cabinets were just luck, but what an instrument.
There is an interview with Sid McGinnis from "Late Night with David Letterman" on Premier guitar where he has a Bassman head upside down for cooling. He makes a comment about the greatest Marshall improvement, flipping the chassis.
The Westinghouse design does not diminish how great a designer Leo Fender was. He made great designs before and kept refining after the Bassman.