I would be interested in seeing any evidence that Morihei Ueshiba taught this particular waza in the Kobukan, before WWII. He probably did, but never gave it a name. Likewise, the waza does not appear among the waza highlighted in Aikido, the book published in Japanese by Kisshomaru Ueshiba in 1957. However, it does appear in the English version of this book, published in 1975, so I suspect that 5-kyou, like kaiten-nage, was added to the repertoire of kihon waza by Kisshomaru Ueshiba.
In my opinion, as a waza, 5-kyou is the same as 1-kyou, except for the hand grip. I have always been taught that 1-kyou is a pin on the elbow; 2-kyou is a pin at the base of the thumb; 3-kyou is a pin of the wrist and outer fingers; 4-kyou is a pin of the bone or nerve centres in the wrist; 5-kyou is a pin on the elbow, but with a weapon held with one hand (unlike 1-kyou, which envisages an attack with a Japanese sword, held with both hands). And then there is 6-kyou, which is a variation of 2-kyou, but where uke's arm is kept straight.
I encountered 6-kyou only in the USA, in Kanai Sensei's dojo, 4 years after my first encounter with aikido.
Best wishes to all,