Bigger, more powerful (especially at height) engines; propellers with 3, then 4, then 5 blades; lengthening the spinner to stop air being lost round propeller blade roots; fully flush-riveted construction; windscreen armour moved inside the windshield; multiple “fishtail” exhaust stubs adding thrust; different mirror housing giving less drag; change from cellulose to synthetic paint, which was smoother; filling and sanding down panel lines and rivet dimples on wing leading edges; adding extra cowling fasteners to stop drag-inducing gaps; employing “Aircraft Finishers” to keep surface finishes as immaculate as possible; higher octane-rated fuel enabling higher boost levels on engines; retractable tailwheels; deletion of aerial wires; eventually replacing aerial mast with a whip aerial.
There were many items, and far more than here.