So the upright, full base twin fin design from 40-ish years ago has become a mainstay for a reason.
It has almost the same surface area as a (smaller) keel twin fin design, but that surface is spread out much more evenly, and vertically. Allowing you to surf your twin more freely (& aggressively) across the wave face as they discovered during the early 80s.
Basically, there's more fin sticking in the water when you put your board on rail, and there's less base than the old skool keel design providing more and earlier release: quicker cornering and a more direct response.
So these fins are a good neutral benchmark or starting point for any twin. We find ourselves going back to these ones lots of times when we encounter fresh twinny shapes that need figuring out...
Here's the specs by True Ames:
Flat Inside Foil: A design from the archives... we were making these fins in the early 80s. This is a true classic twin fin set. A well-balanced and proven design.
HEIGHT: 5.55" / 140 mm
BASE: 5.30" / 135 mm
AREA: 21.67"² / 140.38 cm²