I was looking up something else in the OED when I happened on sense 6.b. of the entry buff, n.2 ‘A buffalo, or other large species of wild ox’:
‘An enthusiast about going to fires’ (Webster 1934); so called from the buff uniforms worn by volunteer firemen in New York City in former times. Hence gen., an enthusiast or specialist. Chiefly N. Amer. colloq.
The first cite for the fire-enthusiast meaning is 1903, the first non-fireman-related one 1931 (Lavine Third Degree vi. 62 “A dentist, known to many cops as a police buff (a person who likes to associate with members of the department and in exchange for having the run of the station house does various courtesies for the police)”). Who knew that the buff of “sports buff” goes back to buffalo, via the buff(alo)-colored uniforms worn by volunteer firemen in New York City? The actual facts of etymology are so much fun I don’t see why people have to resort to imaginary acronyms and the like.