I use the new model BCB flint because the old one, as the name suggests, is no longer available. The one that you describe as the kershaw firestarter is actually a swedish firesteel (and the saw is a laplander saw). The swedish firesteel is, in my opinion to large for a keychain. It would be a good size if you removed the handle bit and drilled a hole through the actual flint (quenching with lots of water).
Hope this is helpful.