Keystone, n. — Green’s Dictionary of Slang

Keystone n.

[Hollywood’s Keystone Cops, a group of comical, incompetent policemen created by director Mack Sennett (1884–1960) in 1912; they featured in a number of films made by his Keystone Studios]

a police officer.

1929		Hostetter & Beesley It’s a Racket! 230: keystone—Public prosecutor for a small town.
1935		A.J. Pollock Und. Speaks n.p.: Keystone, a special, uniformed police officer.
1949		Monteleone Criminal Sl. (rev. edn) 137: key stone A district attorney; a detective.