In my view, it's an adverbial clause. The nominal clause would be The year when two Americans got to the moon was 1969.
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qingqingIt was 1969 when two Americans got to the moon by spaceship.I think it should be rephrased into:
lagatawqingqingIt was 1969 when two Americans got to the moon by spaceship.I think it should be rephrased into:It was in 1969 when two Americans got to the moon by spaceship.(in either case the when-clause here functions as an adverbial clause for me).1969 was the year/time when two Americans got to the moon on a spaceship.(relative