Here's what THE HISTORY OF MARSHALL has to say about the cabinets:
"By 1968 the speaker cloth had changed to the basket-weave style, and the super tall "B" cabinet had been dropped in favour of the rather more practical square design ...
"Assuming that [the] Celestion date code is correct, I have found the heavier duty G12H30s from as early as 1967, though they seem to have been most commonly used from 1969 onwards."
So, it seems to me that the basket weave cabinets were the begining of the new cabinet design, and that the cabinets that would have any real tonal advantage would be the B series cabs from the mid 1960's, using the original Alnico magnets ...
By the time the basket weave cabs were being made, they were already using the ceramic magnets on the Celestions ...
Does that make any sense, Young Angus??