Beneath the Planet of the Apes
It's possible that pre-production work on this film prevented McDowall from reprising his role as Cornelius in "Beneath the Planet of the Apes", the only one of the original five "Planet of the Apes" films from which he is absent. The "Apes" film had finished shooting by 25 June 1969, just two weeks before "Tam-Lin" began.