The luthier filed the outer string grooves deeper to match the 12. He did a good job and it stays in tune. What more can you ask for?
Here are the 120th's specs (nice text formatting!) - it's all stock except for the bridge pup. I have it listed on ebay with a great review of this model.
Gretsch G6118T 120th Anniversary Guitar Features:
MODEL NAME: G6118T-120, Anniversary
MODEL NUMBER: 241-1003-(863)
SERIES: Professional Collection
COLORS: (863) Bamboo Yellow Top with Copper Mist Back Sides and Neck
CATEGORY: Hollow Body Guitars
BODY STYLE: Single Cutaway
SCALE LENGTH: 24.6" (625mm)
TOP: Arched Laminated Maple
BRACING: N/A
BACK AND SIDES: Laminated Maple Body, 16" Wide, 2.5" Deep
NECK: 3-Piece Maple
PICKUPS: TV Jones¨ Classic Filter’Tron / Neck & TV Jones Magna’Tron / Bridge
PICKUP SWITCHING:
3-Position Toggle:
Position 1. Bridge Pickup
Position 2. Bridge and Neck Pickups
Position 3. Neck Pickup
CONTROLS:
Volume 1. (Neck Pickup), Volume 2. (Bridge Pickup), Master Volume,
3-Position Toggle Master Tone Switch:
Position 1. Medium Level, High Frequency Roll Off
Position 2. Switch Out of the Circuit, Pickup is Wide Open (That Great Gretsch Sound!)
Position 3. Slight Level, High Frequency Roll Off
WIDTH AT NUT: 1-11/16" (43mm)
FRETBOARD: Ebony, 12" Radius (305mm)
BRIDGE: Ebony-Based Chrome "Rocking" Bar Bridge
TAILPIECE: Bigsby¨ B6CB Vibrato Tailpiece
HARDWARE: Chrome-Plated
FINISH: Gloss Urethane
CASE: Includes G6241 Deluxe Hardshell Case, US MSRP $220.00
NO. OF FRETS: 21
MACHINE HEADS: Chrome-Plated Grover¨ V98C Sta-Tite Die-cast Tuners
UNIQUE FEATURES: Neo Classic "Thumbnail" Inlay Position Markers, Black Headstock Overlay with Chrome Anniversary Plate, Multiple Body Bindings, Copper Mist Plexi Pickguard, Knurled Strap Retainer Knobs, Adjustable Truss Rod