in the mid 1970s, Cessna POH published operating speeds in CAS, not IAS.
In fact, CAR 3 mandated the use of CAS markings, not IAS, for airspeed indicators. I remember questioning the rationale at the time and could never get anything sensible - I suspect it was one of those things that had "always been done that way" and nobody questioned it.
I believe it was corrected with the introduction of FAR Part 23.