Straight from Greenboy's site:
I am now publishing plans for a superior 215 cab to the authorized builders. I've been calling it "the Dually"... Of all the "traditional" cab configurations, I've always favored the 215 for its better dispersion characteristics, good sensitivity for tube heads, muscular voicing, and fairly high power handling.
But they've always been heavy and bulky. But not now!: with today's better neo drivers - and 1/2" plywood with superior bracing (I really worked it over for this box!) - it's now practical to make a 215 that's easy to move and can really deal out the punishment. The porting here is at the edge of what the best 15" loads for this purpose are capable of.
The target audience is players who are largely traditionalists who would like that type of sound with less weight. By lack of a dedicated midrange driver or a long-excursion woofer like the 3015LF, such a cab is never going to have the flexibility, clarity or upper-scale performance of a 12/6 or 15/6. But it'll definitely do various "old school" "full-range" things louder and lighter than 99% of what's out there.
It's also designed to make it easy to do traditional framed cloth-grills.