Well, a little more digging rustled a serious looking compiliation: Panama! Latin, Calypso and Funk on the Isthmus 1965–1975, which has Los Fabulosos Festivals on it—but not this track.
So they're not such a hidden band afterall, but I came across this serious-looking research paper by Thomas Fawcett (pdf): "The Funky Diaspora: The Diffusion of Soul and Funkacross the Caribbean and Latin America."
A few tastes:
In 1972, a British band made up of nine West Indian immigrants recorded a funk song infused with Caribbean percussion called “The Message.” The band was Cymande, whose members were born in Jamaica, Guyana, and St. Vincent before moving to England between 1958 and 1970.1 In 1973, a year after Cymande recorded “The Message,” the song was reworked by a Panamanian funk band called Los Fabulosos Festivales. The Festivales titled their fuzzed-out, guitar-heavy version “El Mensaje.” A year later the song was covered again, this time slowed down to a crawl and set to a reggae beat and performed by Jamaican singer Tinga Stewart. This example places soul and funk music in a global context and shows that songs were remade, reworked and reinvented across the African diaspora. It also raises issues of migration, language and the power of music to connect distinct communities of the African diaspora.
And,
The principal soul and funk groups of Panama – like the Soul Fantastics, Dinamicos Exciters, and Fabulosos Festivales – grew out of the 1950s combos nacionales. Fusing calypso, cumbia, jazz and doo-wop, the combos were immensely popular four or five man vocal groups made up primarily of black West Indians. In the 1950s, most of the combos were based in Colon, though a few were in Panama City. A group called the Astronautas worked and lived in the U.S. controlled Canal Zone.
Yes, the Internet is useful.