I think what AEO meant to say is that if you have narrow bars you'll want to mount the brake lever right up against the stem. But wider bars will allow you to mount it further away, in the section that has a smaller OD. Personally I don't think this is a big deal and would mount the lever wherever I wanted regardless.

But since your bars have a constant OD that entire point is moot. Are they steel or something? All aluminum bars I've seen have the bigger center section (extending an inch or two on each side) and the rest is smaller. Have you removed the grip tape to check?

In any case, I'd go with the 24mm lever. I won't guarantee it'll fit, but I'm like 95% sure.