The 'Greenbriar Boys' entry in K.Baggelaar & D. Milton's 'The Folk Music Encycopaedia' says that they were formed in 1958 and 'originally comprised John Herald, Bob Yellin and Eric Weissburg'. It goes on to say that Weissburg left the group in 1959 and was replaced by Rinzler. Their information was unually pretty reliable.
The 'Ragged But Right' album sleeve has 'assisted by Eric Weissburg on bass'. The CD 'Best of' is a straight reissue of the Vanguard 'Best of' LP 79159 with no extra tracks - short for a CD, but good though.