Thank you, Keokicat. I have an orginal space pen from Cape Canaveral, that was a promotional item, years ago. It is very plain, the color is mat black with a tiny gold shuttle on the cap. Thought it would be perfect so I dug it out of storage, but it was too fat for the loop in my mini agenda. Bummer. I may try your suggestion and assume it's a skinnier pen since you say it fits. I don't know what the pomme agenda looks like. Is it a mini or does it matter. Are all the pen loops the same size? I'm still a bit green.
My Pomme agenda is the small (PM) size. The spacepen is quite slim. If you need something even thinner, I also bought a pen at Borders. They are almost too thin, though, IMO. I like the spacepen much better.