The IAS must be specified. You might see CAS or simply 'airpseed' in preamble such as specifications but in the part of the AFM where it counts the airspeeds must be indicated airspeed
The design requirement states that Vso is defined in CAS
Sec. 23.49
Stalling speed.
(a)
is the stalling speed, if obtainable, or the minimum steady speed, in miles per hours (CAS), at which the airplane is controllable,................
Therefore, if a pilot is factoring stall speed, the speed being factored should be the CAS speed, and the factor should also be CAS. So, if you want to fly a 1.3Vso approach, a pilot would have to know Vso in IAS. Perhaps the flight manual conveniently provides this. If not, the pilot will have to find it them self by observing IAS at the point of the stall, correcting to CAS with the flight manual table, multiplying by the factor in CAS, obtaining the result, and correcting back to IAS for reference to the airspeed indicator reading. For some types, there can be a 10 knot difference between IAS and CAS at the stall.
I agree that that the use of IAS and CAS can be inconsistent among manufacturer's and types, particularly in the earlier years. And, it would appear that values required to be presented in CAS are being marked on the airspeed indicator in IAS. Once random example of this would be for a 1974, Cessna 185; the POH Airspeed correction table (table 6-1) states "Maximum flap speed 110 MPH, CAS", though in the line above states flaps down IAS is 110, where the CAS is 112. So one would infer that the white line should begin at about 108 MPH to assure that the flaps speed limit is actually observed in CAS. But, on page 4-2, Airspeed limitations, which does state the limits are in CAS, states that the white arc (which is what the in indicating (IAS) pointer shows) is 110 MPH.
Now the 2 MPH difference is not going to create a risk to safety of slight, but it serves to demonstrate the the interchanging of IAS and CAS, even in a POH has been done such that a pilot should really pay attention.
I agree with Chickenhouse that approaches tend to be flown fast, and too fast, more often that at 1.3Vso. Note that 1.3 Vso is a
minimum speed for certification (23.73), the manufacturer may recommend a faster speed. Again referring to the 1974 C 185 (just 'cause I have the book open), full flap stall speed is 56 MPH CAS, X 1.3 = 73 MPH CAS = 69 MPH IAS, so to fly a full flap 1.3 Vso approach, the math would have you at 69 MPH IAS. The POH (page 2-20) says: "For short field landings, make a power off approach at 78 MPH with 40 flaps....". This corresponds to page 1-7, which states landing airspeed of 75 - 85 MPH flaps extended. We have to presume that the POH is presenting these speeds as IAS, though it does not state. Now if that plane has a Horton STOL cuff on the wing, all those speeds will be different, and there is no flight manual supplement to tell the pilot the different speeds (or even a placard saying that they might be different)!