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?