I may have solved the mystery...
Here's a link to the website of a gentleman who published a Master's Degree paper on Ice Cream truck music:
http://homepages.nyu.edu/~dtn9606/ictm.htmlAnd here's a link to his published paper:
http://homepages.nyu.edu/~dtn9606/NeelySoftServe.pdfAccording to the gentleman, the Good Humor melody used in Southern California was based on a Polish (some sources say Czechoslovakian) folk melody called "Stodola Poompa" or "Stodola Pumpa". If you Google "Stodola Pumpa" you can find a couple of MP3's, and it's definitely the same melody as "The Rocking Surfer" and "Come and Get It!"
Lee