Morwen,

I don't understand what you're driving at. King Radio claims to have written the song. He got the copyright registered by 1949 (there's published record of this).

Is that it that you suspect he really didn't write it? Why? FWIW, it sounds to me like a song someone deliberately composed. What reason do you have to suspect it was "traditional" or that Span took it from another source?

Is it just the Grenada/Trinidad/Bahamas discrepancy? Assuming Span was from Trinidad (home of calypso/kaiso), I don't see any reason (yet) to suspect his songs did not gain later popularity on other islands. The song was known in the UK, Canada, and US by 1948:
http://books.google.com/books?id=m5XfAAAAMAAJ&q=%22brown+skin+girl%22&dq=%22brown+skin+girl%22&hl=en&ei=XNWjTYuGPKOE0QH3lbTtCA&sa=X&oi=book_result&ct=result&resnum=7&ved=0CEYQ6AEwBg
http://books.google.com/books?id=vBUbAAAAMAAJ&q=%22brown+skin+girl%22&dq=%22brown+skin+girl%22&hl=en&ei=v9WjTZmqG6mx0QGMxKXuCA&sa=X&oi=book_result&ct=result&resnum=8&ved=0CEoQ6AEwBzgK
http://books.google.com/books?id=w9VKAAAAYAAJ&q=%22brown+skin+gyal%22&dq=%22brown+skin+gyal%22&hl=en&ei=Ld2jTZWsIYWO0QGX8vXrCA&sa=X&oi=book_result&ct=result&resnum=3&ved=0CDcQ6AEwAg

I'm sure the song spread. For instance, Caribbean professional musicians would go to Bahamas to perform for tourists. I don't see anywhere that says the song had to be "native" to a particular island's tradition.

If it was a popular song, by a know artist, what sort of "field recordings" would you expect to be "collected"? Is it that you want other people's non-commercial "cover versions" of King Radio's song?

As for other songs of this type...calypsos from the 1940s!

I hope you don't mistake my tone for rudeness, it is just confusion :) I'm ready to believe there could be an interesting story about this song, but so far I've not seen any reason to suspect there is.