The plane at the beginning of the film flying to Catalina is a Douglas Dolphin 114, registration NC14204. It was one of four airplanes in the Wilmington-Catalina Airlines fleet at the time. It was handed over to the Army in September 1942 and subsequently shipped to Australia. On July 29, 1943 it crashed at Rose Bay, NSW, Australia and was stripped for parts. Only 58 of all variants of the Dolphin were made. The plane was also used in Vivendo em Veludo (1935). Another Dolphin, registration NC967Y is seen at the seaplane ramp on Santa Catalina Island when the group first arrives. That plane was also in Névoa do Mistério (1934).
At the end of Penguin Pool Murder (1932) (the first of three films starring Edna May Oliver and James Gleason), Inspector Oscar Piper proposes to Hildegarde Withers, and they dash out the door to get a marriage license. However, the two characters are not married in either of the two sequels, Sherlock de Saias (1934) and this film. In fact, there's no mention in either movie of an engagement, and the characters give no indication that they've ever been romantically involved.
Third and final film for Edna May Oliver playing Hildegard Withers. The other two films being Penguin Pool Murder (1932) and Sherlock de Saias (1934).
The Joe Slavok aka Red Smith WANTED for MAIL ROBBERY poster that Hildegarde Withers spots on the Post Office wall bears the image of RKO actor Richard Dix, slightly disguised but still easily recognizable. Obviously, this was an inside joke.
While most of the films in the Hildegarde Withers series take place in New York, for this one RKO sent their cast and crew on location to the actual Catalina island just off the coast of Los Angeles. Among the shooting locales were the famous Avalon casino, the docks where tourists arrive on the island and the exterior of the hotel where Miss Withers and Oscar Piper stay while solving the murder around which the plot revolves.