The plate would be pre early 1950s.
Initially, the marks were made up of a local council identifier code of up to 3 letters, followed by a random number, e.g. ABC 123.
In the early 1950s, as numbers started to run out, the components were reversed, giving rise to registrations in the format 123 ABC.
That was until 1963 when the Suffix plates came out.
Not always though, I had a 1964 Morris Minor, with a registration of 770 CAN and that was registered in London in 1964.