As I understand it, the first indegenous Southern Region Loco to recieve a small yellow panel was E6001 in 1962, but it was the only one until 1964, when a start was made to apply them to all units & locos.
The first class 33 to carry a yellow panel was D6580 (the prototype push pull one, still in green) in 1966.
As it goes, the shape of yellow panel on these type 3 locos was still not finalised in 1966, and this first one had rounded corners to it.
So the Southern was two years behind everyone else in making yellow panels standard on their own stock. and even then seemed reluctant to apply yellow to it's stock.
Off topic, but the blue AC electrics of the West Coast Route had no yellow panels until 1965, but then recieved them within weeks throughout the fleet.