Answer:
Found the answer. I think. There was an early case with a flat bottom and from about 1947/8 they had an extra fixing bottom-centre. The 'cooking models' were plain covers and the Star Twin had the piled arms engraved on it.
I ask because I have a 1949 A7 with a plan cover, a spare plain cover and a spare standard engine which has a piled arms cover.
H