Standard practice but illegal, the number plate should be flat and vertical. I've had any number of cars - TVR, Elva, Elan, Piper, and an E-type - with the reg like that but mercifully no trouble from the police. The background should also be unbroken, which scuppers the clever **** who put the numbers and letters on a mesh in the radiator intake of their Elites and Elans.
And, yes, they were painted on before Les Leston produced his stick-ons - which were again illegal because they were easily removable.
What gives you the idea I used to try to make racing cars legal for the road?