Thanks Joe, perhaps I'm confusing myself most of all! Here is an excerpt of the CC51 review that led me this route - note two bolded passages, in hindsight maybe in conflict:
Wear Style: Straight down or over-the-ear
Accessories (4/5) – Single-flange (3 sizes) and bi-flange silicone tips, shirt clip, and hard clamshell carrying case
Build Quality (3.5/5) – The CC51 uses ceramic housings – a first for an earphone readily available in the US (Nakamichi’s ceramics have been available overseas for quite some time). Slightly resembling those of Apple’s dual-driver monitors, the shells of the CC51 are slim and ergonomic. The 6mm driver sits right in the nozzle (as with the JVC and Hippo microdriver earphones)
I guess I walked away from the second passage thinking the nozzle must be something larger than 6mm... I was unsure of whether 6mm listed at the top is a nominal driver size or a direct measurement of the nozzle... but with your clarification as a direct measurement I might try finding a 5.5mm object, forcing a small tip around it, and seeing if that hurts.