Though the third to the last episode aired, this was the final episode of The Wild Wild West (1965) to be filmed.
You'd never guess from her ludicrous accent and cartoonish performance here as the dowager countess, but Nina Foch was once an Oscar nominee for Best Supporting Actress (in Executive Suite). She also had memorable roles in An American in Paris and The Ten Commandments.
When Gordon arrives in the Karovnian camp, he deliberately startles West with a twig snap before following up with a tell-tale bird call. A visibly relieved Jim calls out joyfully to his as-yet unseen partner, "You frightened me, Artie" - a rare admission from the normally unflappable West and a clear inside remark referring to Ross Martin's recent - and VERY real - health scare.
This episode aired one day before Ross Martin's (Artemus Gordon) 49th birthday (born 03-22-1920).