A minor complication arises with some abbreviations. Do you write, "He received a M.A. degree" or "an M.A. degree"? Do you write, "a N.Y. Central spokesman" or "an N.Y. Central spokesman"? The test is how people say or read such designations. "M.A. registers with most people as alphabetical letters, not as "Master of Arts"; hence, "an M.A. degree" is proper.
On the other hand, "N.Y. Central" is instantly translated by the mind into "New York Central"; it would not be read as "En Wye Central." Therefore, a "N.Y. Central spokesman" is proper."
(Theodore M. Bernstein, The Careful Writer: A Modern Guide to English Usage
, Simon & Schuster, 1965) Source