I haven't seen the video but I have a book with a picture of Bukka White playing a National steel guitar. National and Dobro are two companies that make resonator guitars. They were both founded by members of the Dopyera family. Here is a hisory of the two companies. In general, Dobro is better known for the wood body resonator guitars used in bluegrass (though they also make steel bodied guitars) and National is better known for metal body guitars used in blues. Tri-cone refers guitars made by National with three resonator cones inside. They have a distinctive triangular face plate. Tri-cones were made in the 20's and are being reissued by National now. Duolian and Resophonic were, I believe, National trademarks though resophonic is now used as generic term.