Reading the 1955 parts book, the entire set of 4 rocker parts numbers for 235 and 261 do not share a single rocker between engines.
???
My first thought was that the adjuster is different, but it appears not. The shafts match.

There is an interesting engineering feature that is designed in to rocker construction. Almost no factory rockers use "mid-lift", and cannot be adjusted to do this. Instead, a compromise is made to reduce pad-to-stem-tip scrub by lowering the shaft hole in the rocker by a small distance.
The amount varies based on the intended use: it's about 1/3 of the valve lift for passenger car engines (making it about .110" for a 235).
If the rocker were re-designed for low RPM use (as in a 261 truck) the distance would be a greater proportion, about 1/2 of the lift, and with the 261 cam's higher lift the distance would be .205".

Has anyone compared the two rockers?

I was always told the 261 rocker was "stronger".I never noticed a difference,but didn't really look that close.
You mention the 261 being a "low RPM engine".Not really true...Driving old trucks with heavy loads and limited power meant if you needed road speed,the engine was run up against the governor for periods of time.Passanger car engines are almost never run wide open at 3200 rpm for more than a a few seconds.

The Engineering Features brochure from Chevrolet indicates that stiffer rocker arms and shafts were used with 261. See pages 59 and 63 of the linked document. It does not address the shaft offset. I was rooting through my parts collection and could not find any 235 rocker arms to compare to the 261 arms.

Tim Lederman, of the Stovebolt Page, has assembled a most valuable website.

Is there any definitive list of which rockers were used on what, based on the cast-in numbers?
How many different sets are there for the 235 & 261 - 4, or more?

Panic,
A few minutes of looking at other sources shows that 1954 Truck Shop Manual also lists different part numbers for the 235 and 261 engines, but the numbers seem to be different than the numbers in the 1955 shop manual in your post. However, the 1929-1957 Master Parts Catalog shows the same numbers for all 235 and 261 applications. Of course, there are still the four types of rocker arms. The next step would be to look at the rest of the shop and parts manuals, and at a lot of rocker arms.