The bed changed in 1954 - deeper with flat top rails.
As posted above, the same bed dimensions were used from 1954 for many years forward.
However, the bedsides in 1954 were "embossed" around the wheel wells - later years bedsides were flat.
If you want the know many of the changes in 1954, this
1954 Engineering Features booklet provides B&W illustrations and descriptions.
