As for service plates, the older buses such as the Albion
Vikings, Leyland Victories, MB OMF1413s, etc had metal service
plates on the front of the bus.
These only displayed the destination and service numbers.
It was changed to the current style some time in the late 80s,
early 90s.
The metal plates also had different color schemes - red/blue for
aircon, green white for non-aircon and total white (nac) / blue
white (ac) for those services requiring white plates.
During the heydays of the Scania 113 and Volvo B10M MkIIs, there
were 2 service plates on the sides, 1 was for the standard service
plate and the other was used to carry advertisements.