The J channel rests on top of and is bolted to the sub-floor for all early Airstreams. From the 40s to the early 60s, the J channel was formed from sheet aluminum. The extruded U channel came about in 64 or 65. The more complex C/U channel was introduced in 69 for the straight sections and later the curved front and rear.
Work is never done, so take time to play!