Mountains of The Moon is actually in a non-standard tuning. The tuning is as follows: 6=D, 5=G, 4=C, 3=F, 2=A, 1=D. I'm not exactly sure of the name of the tuning but it is the correct tuning for the song, and since Mountains of The Moon is in this tuning Duprees Diamond Blues is in the same tuning. Also in the liner notes from Weir Here Bobby says that Cassidy is in an open tuning that David Crosby showed him. I'm not sure what tuning that would be though. By the way if you listen to the second disc of the remastered Reckoning you can here Bobby tuning down his guitar before the song.Those acoustic Dupree'ses and Mountains of the Moonses from 68 & 69 could very possibly be in non-standard.