Because the stub. I readed a german children the moon book from Dr. Heinz Haber 1969 edition. This book came in short time after the first manned landing on the moon. After apollo 12 a new edition of the book the moon cames in shops. Apollo 21 can also be a mistake by writing of Dr. Heinz Haber. But the apollo-sojus-test-projekt was the 21. flight of the apollo hardware system. Maybe the capsule for ASRP cames from apollo 4 in 1967, but i don`t know it.