I do not have time to study this now, but
there are four different sizes of side rails probably having something to do with truck sizes and/or tire sizes.
[added later: 2 sizes for 53 and two sizes for 54/55st - either two sizes each year for different sizes of trucks and/or for different sizes of wheels?]If someone finds the bottom bracket/brace (that goes on the inside of the bottom bed-side) in the MP&AC, post that info, please. Otherwise, I'll look it up this evening.
There might also be info in the Factory Assembly Manual and/or on Jim Carter's website (he has a Tech Article on the side carrier,
but mixes information from 1954, 1955st, and early Task Force carriers).
