I didn't understand it at all the first time he talked about it, but basically, what it is is a normal guitar with extra frets on the bass end. So that one is basically a normal guitar with six lower frets - so Bb standard.
I really wish he'd play more of the Wardenclyffe Tower material that really showcased the baritones. Unfortunately he's dropped most of it from his regular set list, to avoid bringing multiple guitars on the road.
He actually had three DeLap baritones, all based on the concept Freepower described of adding extra frets to the bottom of a regular 25.5" scale, so he could still use his 8 and 9 gauge string sets with the exact same tension. The smallest went down to C (apparently Carl Verheyen owns that one now), the next went to Bb (which was the one he apparently found most useful), and the largest went down to A, although apparently the bottom fret on that one was about 2 inches wide which even Holdsworth found a bit excessive. He had a piccolo guitar tuned up to A as well, but I've no idea if he ever recorded with that one.
Funny thing is, Uli Jon Roth and Steve Vai apparently had the same idea totally independantly, but never really took it very far... If you look in the guitars section of Vai's website, he has one of his signature guitars with two extra frets on the bottom end to go down an extra tone.
I really wish he'd pursued this concept further, as the results were absolutely stunning.