The very first mission in 1981 or the very first "Return to Flight" in 1988 .....
Maybe.
I found the same image (black and white) at p. 65 of Aerofax Datagraph 5 - "Rockwell International Space Shuttle" - by Dennis Jenkins - 1989 - Aerofax, Inc. ...... with the following (useless) caption: "Each SSME produces 375,000lbs. of thrust at sea level. The engines are regeneratively cooled by liquid hydrogen flowing through thrust chamber walls".