12 P2 saloon identity - 'number under the numberplate'

[credo444]

I early 1980's I rescued a P2 Rover 12 from a scrapyard in Lincolnshire - reg JLT 429. At the time the RSR helped me retain the reg number by identifying it as a very early 1946 model from the chassis plate. However underneath the number plates on both front and rear was another set of 4 numbers, cast in aluminium and rivetted to the numberplate carrier, their condition and fixing suggesting that they had been there from new Having just found some photos of the old car and having the benefit now of web forums I was wondering if anyone could throw any light on the 'number under the plate'.

Andrew