Green’s Dictionary of Slang

(US) a short, heavy revolver with a feature enabling it to double as a knuckleduster.

1927		‘Jargon of the Und.’ in DN V 457: Owl’s head, A short, heavy revolver, frequently having a finger hole in the grip allowing it to be used in addition as a knuckle duster.
1940		‘Digg Mee’ ‘Observation Post’ in N.Y. Age 21 Sept. 10/6: Who was the young man being blitzed at a certain ‘skin-joint’...who went home ot get his ‘owl-head’ and never returned.
1944		T.D. Clark Pills, Petticoats and Plows 130: Competing with the ‘nigger killer’ was the well-known Iver-Johnson ‘owl head’ which was a double-acting piece of unreliable rubber-stocked artillery [DA].
1963		C. Cooper Jr ‘Yet Princes Follow’ in Black! (1996) 271: This surmise gave him a sudden impulse to whip out the Owlhead [...] But he quieted himself.