There is some controversy over the proper plural for RBI, a baseball term meaning "run batted in". Some people say that since the plural of the original phrase would be "runs batted in", which also has the initial letters r, b, and i, the plural of RBI should be RBI. A larger group says the plural should be RBIs (pronounced "Arby eyes").
So I ask the first group: What do you call a single run batted in? Do you call it a RBI or an RBI? If you say "an RBI", you're treating RBI as a new word, not an initialism, so the objection to "RBIs" disappears.
Maybe the best choice is just to follow those who call it a "ribby".