Monogram film, 1946. Spankers are Kane Richmond, who tames Barbara Reed; and George Chandler, who wallops Dorothea Kent. Interestingly, this was Ms. Kent's second screen spanking, exactly 10 years after her first one -- when she was paddled by Jean Arthur (that's right, Anonymous, it was F/F! Horrors!) in Columbia's "More Than a Secretary."
Methinks Ms. Kent rather liked the prone position.
And the reason this scene has never been on "Cinema Swats" is the reason you gave: It is too dark. But hey, I know the guy who produces those things, maybe on your behalf I can talk him into including it next time.