Hi Paul,
Are you talking about a French or Slough built car? (11 or 15)
I think that Slough vehicles changed from lights mounted on the square number plate to wing mounted lights when the change was made to the big boot – 1952.
According to Lucas data, the ST38 (round) was fitted from 1946 to 1952. The rectangular 471 was fitted from 53 -54 on export (USA) cars. Although the list does not seem to say so, as far as I know the 471 was fitted to all UK big boot models.
There has been some debate about the tail lights. Some prefer the originality of the 471, some, like myself, fitted either bigger or extra lights to try to ensure visibility to Audi drivers.
Roger