The Master Tune™ or I lugs (that's a capital i, not a 1) were introduced in 2002 for the new Design Lab Artist Series. From Mapex...

The first truly "Locking Tube Lug", the Master Tune Lug utilizes a small body for the least amount of metal-to-wood contact. A Design Lab-exclusive pressurised locking machanism is outfitted on every lug to keep the drum in tune, show after show.