One of the mysteries around "Mystery In Mexico" is why was the sound credit for Roy Granville credited to Fred L. Granville (Fred LeRoy Granville), who died in 1932. Union issue? Tip of the hat?
Because the budgets of RKO "B" movies had risen from $140,000-$150,000 to the $175,000-$200,000 range, RKO boss Sid Rogell experimented with shooting a movie at Churubusco Studios in Mexico City because RKO had a half interest in it. However, he found that the modest savings from shooting there weren't worth the effort.
This film's earliest documented telecast took place in Memphis Wednesday 25 April 1956 on WHBQ (Channel 13); it first aired in Los Angeles Sunday 19 May 1956 on KHJ (Channel 9), in Salt Lake City Saturday 9 June 1956 on KUTV (Channel 2), in Altoona Monday 25 June 1956 on WFBG (Channel 10), in Philadelphia Tuesday 11 September 1956 on WFIL (Channel 6), in New York City Monday 8 October 1956 on WOR (Channel 9), in Cleveland Thursday 11 October 1956 on KYW (Channel 3), in Sedalia MO Thursday 8 November 1956 on KDRO (Channel 6), in San Francisco Wednesday 5 December 1956 on KPIX (Channel 5), and in Boston Friday 28 December 1956 on WNAC (Channel 7).