nirvana reunion

Last nights Concert for Hurricane Sandy Relief at The Madison Square Garden 12.12.12. found Nirvana's Dave Grohl, Krist Novoselic, and Pat Smear having a reunion with perpetrator from an old band that did actually make *the first grunge recording*.
Fanatics of Nirvana will be outraged and disapproving but the rest of us are wondering what the hell is that square bodied resonator guitar thing called. It sure makes a sweet raucous noise.
*That would be Helter Skelter off the White Album