Different valve lash specs could be related to the type of work done by the engine? Big trucks might have worked at higher RPMs, had higher thermostat temperatures, and might have needed wider valve lash settings?

It is my understanding that 1956 and later 235 & 261 cams were the same cam, matching the 1954/55 261 cam. It looks like the 1957 Master Parts Catalog confirms that understanding?

I think people have checked 1956 and later Shop Manuals, but, I will check a few and report back.