From what I can see in the photos the centre stand currently fitted is the one used on the 1954-1957 frames, part 42 4725. It fits between the mounting lugs.
Looking underneath , when bike is on stand that stand actually bears on the frame cross tube and the welded lugs would not take the later stand which bears on the lugs when bike on stand. This later stand fits outside the mounting lugs.
Going just by the number not letters and the photos your frame, they together, would make it early 1956 season, which would probably have left the factory in late 1955, 1956 frames starting at EA7 101 in September 1955.
The side car mounting on the pillion loop was fitted to 1958/59 FA7 and 1960/63 GA7 frame.