Maybe different-sized panel-body mounting blocks (the wood blocks) were used pre-54 vs 54/55?
That sounds reasonable, but I don't know. I have both a '53 and a 55 1st Suburban, but am not capable of crawling under them right now.
If someone in the Seattle area wants to crawl under my trucks it would be possible to measure them.
I just tried to find the brackets and blocks in the parts catalog and was not successful. Someone not under the influence of the drugs I'm taking these days could probably answer the question that way.
