The original design held the primer in the "percussion module" on the left of the top photo, which was then fed into the chamber essentially by a conventional bolt face like a CF cartridge. The problem was that you needed special tools to prime and deprime the module, and the rifle, because of the conventional bolt design (which was very similar to a bolt action CF) the rifle could be converted to a CF cartridge rifle, which would negate the BATF exemption for muzzleloading arms. Also, if you lost the module in the field, you were screwed.

The 10ML-II eliminated the "percussion module" and replaced it with a fully fixed breech plug design. The 209 primer is now held by the bolt face directly and then cams into the breech plug opening seen on the left of the second picture. This very effectively seals the primer and results in virtually no blowback into the action. Also, the bolt on the 10ML-II is now rear locking and has no front locking lugs, making it very difficult to convert to a CF rifle.

Mike