From Tim O'Brien's web site:

Tim's Guitar - A Collings Dreadnaught

This slotted headstock dreadnaught is made by Collings Guitars in Austin Texas. I've always loved the look and the sound of this type of instrument. The sloping shoulders disguise the fact that the 25 and 3/4 inch scale (from nut to bridge saddle) is the same length as normal dreadnaughts. The fingerboard meets the body at the 12th fret instead of the more normal 14th fret. Sure it's harder to play up past the twelfth fret, but as a lot of country players know, there's not much money past the 5th fret anyway! The advantage to this design is a little more top surface, as well as the increased angle behind the nut that the slotted headstock features. It has a wide fingerboard, which I love for finger style playing. Collings small factory makes some of the finest acoustic guitars you can purchase these days. They are also making some fine mandolins. In fact a Tim O'Brien model is in the works - watch for them to be available in limited production very soon. It will be a black top A-5 mandolin, in the style of my old standby Nugget mandolin, with custom fine tuning and finish by Mike Kemnitzer of Nugget mandolins.