The meaning of the line colours and styles changed at some point ...
From memory they changed around 1974 when we went from red covered inch to a mile maps to magenta covered 2cm to a km maps.
First Series Metric Landrangers were just the old 1 inch map base enlarged.
I think the A - Red, B-Brown, Yellow, colouring came in in the 1940s reprints of the New Popular Editions.
Hold on I should be able to get the Key from the NLS map images