Mk I starting early in 1975 w/ the March S.N.A.C.K. show ( I figure it probable it appeared earlier w/ Legion of Mary, maybe even November-December '74? ), thru Detroit 10/3/76, the next shows being Oakland on the Green w/ the debut of the dual Twin unit.
Mk I also appears in undated (but 1979) Reconstruction photo on the jerrysite.com
and then like he said, Feb-Mar 80 JGB w/ the Mk IIa.
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia