That's an excellent suggestion! :p I had no idea these tools existed. It looks like Irwin even makes an adjustable one like a drill chuck. I think you may have just solved my problem!
So it appears the overall assessment is right, there is no such animal as an E9. Followed the two provided links or searches, and though the products list a range of these sockets, when the full list of exact included tools is examined, E9 is not included. Neither are many larger sizes.
I...