I came here after having purchased a copy of "The Dictionary Companion" and Googling the author. I came across a New York Times obituary from 1938 (Saturday, Nov. 5, page 19):
C. O. SYLVESTER MAWSON
BOSTON, Nov. 4 (AP) — C.O. Sylvestor Mawson, well-known lexicographer, suffered a heart attack tonight while motoring with his wife and died just after he brought his car to a stop. His age was 66.