Bastel is right.
It looks like the "Road-Tax-License"-Holder,
normally used in Great Britain. I had such
a thing at my Ölfield-Bullshit, bought in
London many years ago.
I think, you will find those parts easily in
England. Or in Gibraltar...
Further-Cruising,
Bullet-Pit
("Made like a gun")
More infos here. Guckst you:
https://en.wikipedia.org/wiki/Vehicle_Excise_Duty