Ikkyo does not equal Ikkajo
We all agree that aikido was born out of the Daito ryuha form of unarmed combat namely daito ryu aiki jujutsu. We also know that aiki jujutsu uses the terms ikkajo to gokajo. What needs to be realised is that these terms are simply that, terms. They are not names of techniques. Ikkajo to gokajo are the "Shiden Mokuroku", the first 118 techniques of aiki jujutsu.
Ikkajo comprises of 30 techniques one of which, ippon dori resembles what we know as ikkyo. It is called Ippon dori within aiki jujutsu. Nikajo also comprises of 30 techniques, as does Sankajo. Yonkajo comprises of 15 techniques and gokajo comprises of 13 techniques. These being the 118 of the Shiden mokuroku.
As far as I am aware each of the "jo's" has within it a technique similar to the technique that we would recognise as being ikkyo for example. However they also contain vastly different techniques for example ikkajo contains koshi guruma what we call koshi nage and gokajo contains shimoku what we call ganseki otoshi. As to the so-called missing techniques I cannot comment other than to say that rokajo etc are not found within the Daito ryu aikijujutsu curriculum.
Again I cannot say why the Yoshinkan use the names ikkajo-gokajo for their first five techniques. Perhaps it is a throw back to Takeda sensei or perhaps O'sensei decided only to teach these forms from the sets.
What is clear though is that someone systemised our techniques as they are generally very cut and dry Japanese descriptions or rather flowery ones, i.e. kote gaeshi, hiji kime osae all the way to tenchi nage and ganseki otoshi.
I have heard sensei use cut and dry terms such as ude osae, kote hineri, kote mawashi etc etc for our first five osae waza. It is a very intriguing question one that deserves proper academic research. But I restate the need to not draw our own conclusion without firm evidence to so.