Follower and base plate is my best guess. Looking at the ETS pro clear mags you can see where they get the two extra rounds. I modified one my spare p30sk 10rd mags to easily accept 11rds. "Easily", because it's very close to a 12th rd. I can see where to get more room, but it's more modification in harder to reach places.
The real question is: Will HK make the followers and base plates available to us?