Every where I look, people use the terms Ikkyo, nikyo, sankyo, yonkyo, etc.
In my style of aikido (Yonshinkai) we use the terms Ikkajo which literary means first control (or so I've heard.) It is the same thing for nikajo, sankajo, yonkajo...
I speak no Japanese, but....
There's an interview with a Daito Ryu sensei on the aikidofaq where he talks a little about this.(Well, it's related...) I think in daito ryu Ikkajo refers to about thirty techniques, and this guy was saying in this interview that one of these became Ikkyo. I doubt it's so cut and dried, and the difference is probably about a different approach to teaching emphasis. Maybe.
Round here we translate "Ikkyo" as "first principle."
Yonshinkan developed seperately from Aikikai from the thirties, did it not? The names "Ikkyo.." etc. probably weren't in use when Gozo Shioda got going...