"The Secrets of Tracy Island" is a short story published in TV Century 21's 1967 Thunderbirds Spring Extra.
The story is a first person recount by Jeff Tracy of how International Rescue came to be, written from several years after the fact. Keeping in line with the 'in-universe' perspective of the article, and the TV Century 21 shared universe overall, the recount is said to be a top-secret document, issued by the Universal Secret Service for the eyes of Contact 21 agents only.
This particular version of International Rescue's formation is the most obscure telling of the story, being eclipsed by The Complete Thunderbirds Story and, to a lesser degree, Countdown To Action!. This is partly due to the fact it appeared in an "extra" publication, and has never been reprinted. However, this story would form the basis of The Complete Thunderbirds Story, meaning it would likely be Alan Fennell that penned this version. Such similarities would include the specific failures of the Thunderbirds during construction, with Thunderbird 1 crashing and almost killing Scott, TB2 dropping a pod onto the Tracy Villa, and TB4 suffering a buoyancy fault that trapped Gordon on the sea floor.