The original Series1 Heron had fixed gear and two bladed fixed pitch props.
Series 2 still had the two blades, but variable pitch with NO pitch control (or mixture for that matter) on the quadrant. T'was automagic.
That lead some to assume that they only had fixed pitch also.
The Twin Commical was a bit higgeldy in the A and B models, but they had it pretty well sorted out in the C onwards.