According to:
https://www.airspacemag.com/need-to-...ter-108981404/
...square tips are more effective per radius - up to the point (airspeed + tip speed) where they develop "compressibility burble" sooner than rounded/elliptical tips.
C-130 is slower, and needs the extra prop "push" on takeoffs, from the square planform. CP-140 has a higher cruise speed (405 kt vs. 330 kt) where the burble is more of a problem.
As to the Electra - I see a (very)
few pictures of them with rounded prop tips. Probably just one of those "Oh, why didn't we think of that" changes made over time, and as the airframe was updated (e.g. CP-140) over decades.
http://imgproc.!!!!!!!!!!!!!!/photos/...4834.jpg?v=v40