Yeah, the letter from the factory this late in production is pretty remarkable...You'd expect by then, that a real 306hp 289 was a pretty precious item to DeT, but when selling to the son of the President of Mexico, a pretty astute move...
But here, its unusual that a 289 car may have been advertised as a 302--when its usually the opposite sin (sellers use mythology that claims all Euro cars were 289 Hi-Pos and then claim their cars have 306 HP...).
Definitely, the parts manual points to many "solo 289" (289 specific pieces (like jackshaft housing) and the non-smog bell housing was used for ROW.
Pictures on Provamo show it with a Holley and non-original valve covers...geez, who in the 1960s wouldn't have wanted to hot rod a Ford, its amazing any cars have original intake/carb or even valve covers...Lee