Tim,
Your comments are all well-taken.
However, your general comments regarding 235/261 springs and compressed spring-heights being the same is only correct in later years (as I posted), but the springs and spring heights were not the same in earlier years of the 261.
And, yes, GM indicated in 1954 that the rotators were used/introduced on/in the 261 (only in/on the 261 in 1954 for a few years) to increase valve life/durability.
