Aside from battery suppliers in Barbados, Bangalore, Jamaica and Jakarta, I can’t seem to find out what distinguishes a tropical battery from a standard one. Perhaps one that self-fills the cells? Is that a thing? Sealed batteries were probably not around 50 years ago.
More humorously, the thought of a car destined for Milwaukee (in the frozen tundra of the upper Great Lakes) needing a “tropical” battery is a bit funny. If anything I’d think it would have needed one with a higher CCA rating than standard.