Here are additional covers from the 1977 time frame.
The Space Shuttle Enterprise was rollout at Rockwell International, Space Division, Palkmdale, Ca. on September 17, 1976.

Cover was autographed by the following:
(1) Program Director at NASA Headquarters, Myron Malkin
(2) Project Manager at KSC, Dr. Robery Gray
(3) Orbiter Project Manager at JSC, Aaron Cohen
(4) Program Manager at JSC, Robert Thompson
(5) Approach and Landing Test Manager, Deke Slayton
(6) Project manager at MSFC, Robert Lindstrom
(7) ET Project Manager at MSFC, James Odom
(8) SRB Project Manager at MSFC, George Hardy
(9) SSME Project Manager at MSFC, James Thompson