I've had and played with this Leyland petrol tanker since I was a kid. Another one I'd passed onto my son only to google it and not be able to find it anywhere.
The tank has been overpainted. (If you look at the chipped paint edge very carefully, you can see that the black paint is overlying the original white.)
I've always thought it might be. The thing that makes me wonder if not is that it is there is no paint over the green and the front of the tanker is is also black while the ccab is untouched.
Considering how bad the 203 looks, it's amazing to me someone could paint that and yet also paint the front with such accuracy..