In the film, the deaf-mute young man, who passes the message to the contract killer, is shown reading a comic book. The cover (featuring Superman and Lois Lane) clearly identifies the issue as Superman #27. The release date of this comic was about January 5, 1944, so the scene could not have been shot before that date. And that date is probably too early, since comics were generally printed in the New Jersey area and then had to be shipped by train or truck around the country. It is likely that copies would not have appeared on Hollywood newsstands until the 8th or later, perhaps not until the 12th. Of course, the scene could have been shot several weeks later than that, as Superman was bimonthly at the time and new copies of that issue could have been found on the stands up to the first week of March 1944. However, a March date for filming the scene is not likely, since according to IMDb the finished film was released on March 30, 1944. So that scene was probably filmed between January 8th and February 29th, 1944.
There were eight entries in the "Whistler" series, and Richard Dix appeared in all but the last one. Unusually, he played a different character in each.
The signature whistling at the beginning of all The Whistler movies was provided by Dorothy Roberts, backed by the theme's composer Wilbur Hatch and his orchestra.
By the time Richard Dix started the Whistler movies, he was a heavy drinker and subject to hiccups.
This film series is based on the radio program "The Whistler" that aired on the CBS radio network from May 16, 1942 until September 22, 1955.