The Ryōi Shintō-ryū (良移心当流) - Atemi no Maki (当身之巻 - scroll of striking) features ‘Ryōken’ (菱拳), or alternatively ‘Hishi Kobushi’ (菱拳). The translation for which is ‘Diamond shaped fist’, or ‘Water chestnut fist’ and this utilises a protruding middle knuckle. 

In Chinese Martial Arts there is ‘Tiě líng quán’ (鉄菱拳), which means ‘Iron diamond  / water chestnut fist’ and this is also done also with a protruding middle knuckle.

The accompanying picture (taken from Shortt & Hashimoto 1979) shows the extended knuckle fist striking the floating ribs’. 

On the left the point would be 'Tsukikage' (月影 Moonbeams) and on the  right Denkō’ (電光 - Lightning) 

Superficially both points can used to attack the ribs, intercostal muscles and nerves, but on a deeper level structures such as the liver and diaphragm.

(My thanks to Logan Weymouth for his assistance in translating from Japanese to English)